diff --git a/.sobelow-skips b/.sobelow-skips
index 70978cfa5..28a98366b 100644
--- a/.sobelow-skips
+++ b/.sobelow-skips
@@ -7,4 +7,5 @@
 155A1FB53DE39EC8EFCFD7FB94EA823D
 73B351E4CB3AF715AD450A085F5E6304
 BBACD7F0BACD4A6D3010C26604671692
-6D4D4A4821B93BCFAC9CDBB367B34C4B
\ No newline at end of file
+6D4D4A4821B93BCFAC9CDBB367B34C4B
+5674F0D127852889ED0132DC2F442AAB
\ No newline at end of file
diff --git a/js/src/utils/i18n.ts b/js/src/utils/i18n.ts
index 5e2231c0f..c772484d7 100644
--- a/js/src/utils/i18n.ts
+++ b/js/src/utils/i18n.ts
@@ -39,9 +39,19 @@ const loadedLanguages = [DEFAULT_LOCALE];
 
 function setI18nLanguage(lang: string): string {
   i18n.locale = lang;
+  setLanguageInDOM(lang);
   return lang;
 }
 
+function setLanguageInDOM(lang: string): void {
+  const fixedLang = lang.replaceAll("_", "-");
+  const html = document.documentElement;
+  const documentLang = html.getAttribute("lang");
+  if (documentLang !== fixedLang) {
+    html.setAttribute("lang", fixedLang);
+  }
+}
+
 function fileForLanguage(matches: Record<string, string>, lang: string) {
   if (Object.prototype.hasOwnProperty.call(matches, lang)) {
     return matches[lang];
diff --git a/lib/graphql/error.ex b/lib/graphql/error.ex
index 653a09051..e6b87b2c8 100644
--- a/lib/graphql/error.ex
+++ b/lib/graphql/error.ex
@@ -5,7 +5,8 @@ defmodule Mobilizon.GraphQL.Error do
 
   require Logger
   alias __MODULE__
-  import Mobilizon.Web.Gettext
+  alias Mobilizon.Web.Gettext, as: GettextBackend
+  import Mobilizon.Web.Gettext, only: [dgettext: 2]
 
   defstruct [:code, :message, :status_code, :field]
 
@@ -44,7 +45,7 @@ defmodule Mobilizon.GraphQL.Error do
 
   defp handle(%Ecto.Changeset{} = changeset) do
     changeset
-    |> Ecto.Changeset.traverse_errors(fn {err, _opts} -> err end)
+    |> Ecto.Changeset.traverse_errors(&translate_error/1)
     |> Enum.map(fn {k, v} ->
       %Error{
         code: :validation,
@@ -96,4 +97,27 @@ defmodule Mobilizon.GraphQL.Error do
     Logger.warn("Unhandled error code: #{inspect(code)}")
     {422, to_string(code)}
   end
+
+  # Translates an error message using gettext.
+  defp translate_error({msg, opts}) do
+    # Because error messages were defined within Ecto, we must
+    # call the Gettext module passing our Gettext backend. We
+    # also use the "errors" domain as translations are placed
+    # in the errors.po file.
+    # Ecto will pass the :count keyword if the error message is
+    # meant to be pluralized.
+    # On your own code and templates, depending on whether you
+    # need the message to be pluralized or not, this could be
+    # written simply as:
+    #
+    #     dngettext "errors", "1 file", "%{count} files", count
+    #     dgettext "errors", "is invalid"
+    #
+
+    if count = opts[:count] do
+      Gettext.dngettext(GettextBackend, "errors", msg, msg, count, opts)
+    else
+      Gettext.dgettext(GettextBackend, "errors", msg, opts)
+    end
+  end
 end
diff --git a/lib/graphql/resolvers/actor.ex b/lib/graphql/resolvers/actor.ex
index 54e60293e..c1279d9b9 100644
--- a/lib/graphql/resolvers/actor.ex
+++ b/lib/graphql/resolvers/actor.ex
@@ -9,7 +9,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Actor do
   alias Mobilizon.Federation.ActivityPub
   alias Mobilizon.Service.Workers.Background
   alias Mobilizon.Users.User
-  import Mobilizon.Web.Gettext
+  import Mobilizon.Web.Gettext, only: [dgettext: 2]
 
   require Logger
 
diff --git a/lib/mobilizon/config.ex b/lib/mobilizon/config.ex
index 600ae63c8..c4e66c7bd 100644
--- a/lib/mobilizon/config.ex
+++ b/lib/mobilizon/config.ex
@@ -348,8 +348,7 @@ defmodule Mobilizon.Config do
   end
 
   def generate_terms(locale) do
-    import Mobilizon.Web.Gettext
-    put_locale(locale)
+    Gettext.put_locale(locale)
 
     Phoenix.View.render_to_string(
       Mobilizon.Web.APIView,
@@ -363,8 +362,7 @@ defmodule Mobilizon.Config do
   end
 
   def generate_privacy(locale) do
-    import Mobilizon.Web.Gettext
-    put_locale(locale)
+    Gettext.put_locale(locale)
 
     Phoenix.View.render_to_string(
       Mobilizon.Web.APIView,
diff --git a/lib/service/activity/renderer/comment.ex b/lib/service/activity/renderer/comment.ex
index 87f62796c..c7551c6d2 100644
--- a/lib/service/activity/renderer/comment.ex
+++ b/lib/service/activity/renderer/comment.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Comment do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/discussion.ex b/lib/service/activity/renderer/discussion.ex
index 11328ed85..8c1d58f4f 100644
--- a/lib/service/activity/renderer/discussion.ex
+++ b/lib/service/activity/renderer/discussion.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Discussion do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/event.ex b/lib/service/activity/renderer/event.ex
index 7c8eb5993..809583bb0 100644
--- a/lib/service/activity/renderer/event.ex
+++ b/lib/service/activity/renderer/event.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Event do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/group.ex b/lib/service/activity/renderer/group.ex
index 15f7a66b9..a9df9d826 100644
--- a/lib/service/activity/renderer/group.ex
+++ b/lib/service/activity/renderer/group.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Group do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/member.ex b/lib/service/activity/renderer/member.ex
index 9581a1a92..64bdda534 100644
--- a/lib/service/activity/renderer/member.ex
+++ b/lib/service/activity/renderer/member.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Member do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/post.ex b/lib/service/activity/renderer/post.ex
index 4f92e4469..1491e58e8 100644
--- a/lib/service/activity/renderer/post.ex
+++ b/lib/service/activity/renderer/post.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Post do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/activity/renderer/resource.ex b/lib/service/activity/renderer/resource.ex
index 31f7ccfcd..4e328b236 100644
--- a/lib/service/activity/renderer/resource.ex
+++ b/lib/service/activity/renderer/resource.ex
@@ -5,7 +5,7 @@ defmodule Mobilizon.Service.Activity.Renderer.Resource do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Renderer
-  alias Mobilizon.Web.{Endpoint, Gettext}
+  alias Mobilizon.Web.Endpoint
   alias Mobilizon.Web.Router.Helpers, as: Routes
   import Mobilizon.Web.Gettext, only: [dgettext: 3]
 
diff --git a/lib/service/metadata/utils.ex b/lib/service/metadata/utils.ex
index 36416919d..2e1992dfb 100644
--- a/lib/service/metadata/utils.ex
+++ b/lib/service/metadata/utils.ex
@@ -6,7 +6,7 @@ defmodule Mobilizon.Service.Metadata.Utils do
   alias Mobilizon.Service.{Address, DateTime}
   alias Mobilizon.Service.Formatter.HTML, as: HTMLFormatter
   alias Phoenix.HTML
-  import Mobilizon.Web.Gettext
+  import Mobilizon.Web.Gettext, only: [gettext: 1]
 
   @slice_limit 200
 
diff --git a/lib/web/auth/context.ex b/lib/web/auth/context.ex
index 468a0d692..68a1c4ffe 100644
--- a/lib/web/auth/context.ex
+++ b/lib/web/auth/context.ex
@@ -22,17 +22,19 @@ defmodule Mobilizon.Web.Auth.Context do
   def set_user_information_in_context(conn) do
     context = %{ip: conn.remote_ip |> :inet.ntoa() |> to_string()}
 
-    context =
+    {conn, context} =
       case Guardian.Plug.current_resource(conn) do
         %User{id: user_id, email: user_email} = user ->
           if SentryAdapter.enabled?() do
             Sentry.Context.set_user_context(%{id: user_id, name: user_email})
           end
 
-          Map.put(context, :current_user, user)
+          context = Map.put(context, :current_user, user)
+          conn = assign(conn, :user_locale, user.locale)
+          {conn, context}
 
         nil ->
-          context
+          {conn, context}
       end
 
     context =
diff --git a/lib/web/email/activity.ex b/lib/web/email/activity.ex
index 339fcb9eb..17268424c 100644
--- a/lib/web/email/activity.ex
+++ b/lib/web/email/activity.ex
@@ -10,7 +10,7 @@ defmodule Mobilizon.Web.Email.Activity do
   alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Config
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @spec direct_activity(String.t(), list(), String.t()) ::
           Bamboo.Email.t()
diff --git a/lib/web/email/admin.ex b/lib/web/email/admin.ex
index 782c32709..bde93cf0d 100644
--- a/lib/web/email/admin.ex
+++ b/lib/web/email/admin.ex
@@ -13,7 +13,7 @@ defmodule Mobilizon.Web.Email.Admin do
   alias Mobilizon.Reports.Report
   alias Mobilizon.Users.User
 
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @spec report(User.t(), Report.t(), String.t()) :: Bamboo.Email.t()
   def report(%User{email: email} = user, %Report{} = report, default_locale \\ "en") do
diff --git a/lib/web/email/event.ex b/lib/web/email/event.ex
index 5b3e39ddc..b49a3baae 100644
--- a/lib/web/email/event.ex
+++ b/lib/web/email/event.ex
@@ -16,7 +16,6 @@ defmodule Mobilizon.Web.Email.Event do
   alias Mobilizon.Users.{Setting, User}
 
   alias Mobilizon.Web.Email
-  alias Mobilizon.Web.Gettext, as: GettextBackend
 
   @important_changes [:title, :begins_on, :ends_on, :status, :physical_address]
 
@@ -31,7 +30,7 @@ defmodule Mobilizon.Web.Email.Event do
         timezone \\ "Etc/UTC",
         locale \\ "en"
       ) do
-    GettextBackend.put_locale(locale)
+    Gettext.put_locale(locale)
 
     subject =
       gettext(
diff --git a/lib/web/email/follow.ex b/lib/web/email/follow.ex
index 25c3c953d..b6c1a523f 100644
--- a/lib/web/email/follow.ex
+++ b/lib/web/email/follow.ex
@@ -11,7 +11,7 @@ defmodule Mobilizon.Web.Email.Follow do
   alias Mobilizon.Actors.{Actor, Follower}
   alias Mobilizon.Federation.ActivityPub.Relay
   alias Mobilizon.Users.User
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @doc """
   Send follow notification to admins if the followed actor
diff --git a/lib/web/email/group.ex b/lib/web/email/group.ex
index 2c3029007..8e1e938ec 100644
--- a/lib/web/email/group.ex
+++ b/lib/web/email/group.ex
@@ -10,7 +10,7 @@ defmodule Mobilizon.Web.Email.Group do
   alias Mobilizon.{Actors, Config, Users}
   alias Mobilizon.Actors.{Actor, Member}
   alias Mobilizon.Users.User
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @doc """
   Send emails to local user
diff --git a/lib/web/email/notification.ex b/lib/web/email/notification.ex
index e27b13efb..5800bf0f4 100644
--- a/lib/web/email/notification.ex
+++ b/lib/web/email/notification.ex
@@ -9,7 +9,7 @@ defmodule Mobilizon.Web.Email.Notification do
 
   alias Mobilizon.Events.{Event, Participant}
   alias Mobilizon.Users.{Setting, User}
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @spec before_event_notification(String.t(), Participant.t(), String.t()) ::
           Bamboo.Email.t()
diff --git a/lib/web/email/participation.ex b/lib/web/email/participation.ex
index dbd90e694..edf319906 100644
--- a/lib/web/email/participation.ex
+++ b/lib/web/email/participation.ex
@@ -12,7 +12,7 @@ defmodule Mobilizon.Web.Email.Participation do
   alias Mobilizon.Events.{Event, Participant}
   alias Mobilizon.Users
   alias Mobilizon.Users.User
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   @doc """
   Send participation emails to local user
diff --git a/lib/web/email/user.ex b/lib/web/email/user.ex
index 7dd74b70b..fdbdcdf79 100644
--- a/lib/web/email/user.ex
+++ b/lib/web/email/user.ex
@@ -7,13 +7,13 @@ defmodule Mobilizon.Web.Email.User do
 
   import Bamboo.Phoenix
 
-  import Mobilizon.Web.Gettext
+  import Mobilizon.Web.Gettext, only: [gettext: 2]
 
   alias Mobilizon.{Config, Crypto, Users}
   alias Mobilizon.Storage.Repo
   alias Mobilizon.Users.User
 
-  alias Mobilizon.Web.{Email, Gettext}
+  alias Mobilizon.Web.Email
 
   require Logger
 
diff --git a/lib/web/endpoint.ex b/lib/web/endpoint.ex
index 286ab9778..2b1455551 100644
--- a/lib/web/endpoint.ex
+++ b/lib/web/endpoint.ex
@@ -12,7 +12,7 @@ defmodule Mobilizon.Web.Endpoint do
   use Phoenix.Endpoint, otp_app: :mobilizon
   use Absinthe.Phoenix.Endpoint
 
-  plug(Mobilizon.Web.Plugs.SetLocalePlug)
+  plug(Mobilizon.Web.Plugs.DetectLocalePlug)
 
   if Application.fetch_env!(:mobilizon, :env) !== :dev do
     plug(Mobilizon.Web.Plugs.HTTPSecurityPlug)
diff --git a/lib/web/gettext.ex b/lib/web/gettext.ex
index 9f9604772..055113a85 100644
--- a/lib/web/gettext.ex
+++ b/lib/web/gettext.ex
@@ -21,30 +21,4 @@ defmodule Mobilizon.Web.Gettext do
   See the [Gettext Docs](https://hexdocs.pm/gettext) for detailed usage.
   """
   use Gettext, otp_app: :mobilizon
-
-  def put_locale(locale) do
-    locale = determine_best_locale(locale)
-    Gettext.put_locale(__MODULE__, locale)
-  end
-
-  @spec determine_best_locale(String.t()) :: String.t()
-  def determine_best_locale(locale) do
-    locale = String.trim(locale)
-    locales = Gettext.known_locales(__MODULE__)
-    default = Keyword.get(Mobilizon.Config.instance_config(), :default_language, "en") || "en"
-
-    cond do
-      # Default if nothing provided
-      locale == "" -> default
-      # Either it matches directly, eg: "en" => "en", "fr" => "fr"
-      locale in locales -> locale
-      # Either the first part matches, "fr_CA" => "fr"
-      split_locale(locale) in locales -> split_locale(locale)
-      # Otherwise set to default
-      true -> default
-    end
-  end
-
-  # Keep only the first part of the locale
-  defp split_locale(locale), do: locale |> String.split("_", trim: true, parts: 2) |> hd
 end
diff --git a/lib/web/mobilizon_web.ex b/lib/web/mobilizon_web.ex
index fa691357b..76f5b5fe1 100644
--- a/lib/web/mobilizon_web.ex
+++ b/lib/web/mobilizon_web.ex
@@ -40,7 +40,6 @@ defmodule Mobilizon.Web do
       use Phoenix.HTML
 
       import Mobilizon.Web.Router.Helpers
-      import Mobilizon.Web.ErrorHelpers
       import Mobilizon.Web.Gettext
     end
   end
diff --git a/lib/web/plugs/detect_locale_plug.ex b/lib/web/plugs/detect_locale_plug.ex
new file mode 100644
index 000000000..66c339aca
--- /dev/null
+++ b/lib/web/plugs/detect_locale_plug.ex
@@ -0,0 +1,67 @@
+# Portions of this file are derived from Pleroma:
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+# NOTE: this module is based on https://github.com/smeevil/set_locale
+defmodule Mobilizon.Web.Plugs.DetectLocalePlug do
+  @moduledoc """
+  Plug to set locale for Gettext
+  """
+  import Plug.Conn, only: [get_req_header: 2, assign: 3]
+  alias Mobilizon.Web.Gettext, as: GettextBackend
+
+  def init(_), do: nil
+
+  def call(conn, _) do
+    locale = get_locale_from_header(conn)
+    assign(conn, :detected_locale, locale)
+  end
+
+  defp get_locale_from_header(conn) do
+    conn
+    |> extract_accept_language()
+    |> Enum.find(&supported_locale?/1)
+  end
+
+  defp extract_accept_language(conn) do
+    case get_req_header(conn, "accept-language") do
+      [value | _] ->
+        value
+        |> String.split(",")
+        |> Enum.map(&parse_language_option/1)
+        |> Enum.sort(&(&1.quality > &2.quality))
+        |> Enum.map(& &1.tag)
+        |> Enum.reject(&is_nil/1)
+        |> ensure_language_fallbacks()
+
+      _ ->
+        []
+    end
+  end
+
+  defp supported_locale?(locale) do
+    GettextBackend
+    |> Gettext.known_locales()
+    |> Enum.member?(locale)
+  end
+
+  defp parse_language_option(string) do
+    captures = Regex.named_captures(~r/^\s?(?<tag>[\w\-]+)(?:;q=(?<quality>[\d\.]+))?$/i, string)
+
+    quality =
+      case Float.parse(captures["quality"] || "1.0") do
+        {val, _} -> val
+        :error -> 1.0
+      end
+
+    %{tag: captures["tag"], quality: quality}
+  end
+
+  defp ensure_language_fallbacks(tags) do
+    Enum.flat_map(tags, fn tag ->
+      [language | _] = String.split(tag, "-")
+      if Enum.member?(tags, language), do: [tag], else: [tag, language]
+    end)
+  end
+end
diff --git a/lib/web/plugs/set_locale_plug.ex b/lib/web/plugs/set_locale_plug.ex
index 8bcc83324..0af554e13 100644
--- a/lib/web/plugs/set_locale_plug.ex
+++ b/lib/web/plugs/set_locale_plug.ex
@@ -8,61 +8,55 @@ defmodule Mobilizon.Web.Plugs.SetLocalePlug do
   @moduledoc """
   Plug to set locale for Gettext
   """
-  import Plug.Conn, only: [get_req_header: 2, assign: 3]
+  import Plug.Conn, only: [assign: 3]
   alias Mobilizon.Web.Gettext, as: GettextBackend
 
   def init(_), do: nil
 
   def call(conn, _) do
-    locale = get_locale_from_header(conn)
-    GettextBackend.put_locale(locale)
+    locale =
+      [
+        conn.assigns[:user_locale],
+        conn.assigns[:detected_locale],
+        default_locale(),
+        "en"
+      ]
+      |> Enum.map(&determine_best_locale/1)
+      |> Enum.filter(&supported_locale?/1)
+      |> hd()
+
+    Gettext.put_locale(locale)
     assign(conn, :locale, locale)
   end
 
-  defp get_locale_from_header(conn) do
-    conn
-    |> extract_accept_language()
-    |> Enum.find("", &supported_locale?/1)
-  end
-
-  defp extract_accept_language(conn) do
-    case get_req_header(conn, "accept-language") do
-      [value | _] ->
-        value
-        |> String.split(",")
-        |> Enum.map(&parse_language_option/1)
-        |> Enum.sort(&(&1.quality > &2.quality))
-        |> Enum.map(& &1.tag)
-        |> Enum.reject(&is_nil/1)
-        |> ensure_language_fallbacks()
-
-      _ ->
-        []
-    end
-  end
-
   defp supported_locale?(locale) do
     GettextBackend
     |> Gettext.known_locales()
     |> Enum.member?(locale)
   end
 
-  defp parse_language_option(string) do
-    captures = Regex.named_captures(~r/^\s?(?<tag>[\w\-]+)(?:;q=(?<quality>[\d\.]+))?$/i, string)
-
-    quality =
-      case Float.parse(captures["quality"] || "1.0") do
-        {val, _} -> val
-        :error -> 1.0
-      end
-
-    %{tag: captures["tag"], quality: quality}
+  defp default_locale do
+    Keyword.get(Mobilizon.Config.instance_config(), :default_language, "en")
   end
 
-  defp ensure_language_fallbacks(tags) do
-    Enum.flat_map(tags, fn tag ->
-      [language | _] = String.split(tag, "-")
-      if Enum.member?(tags, language), do: [tag], else: [tag, language]
-    end)
+  @spec determine_best_locale(String.t()) :: String.t()
+  def determine_best_locale(locale) when is_binary(locale) do
+    locale = String.trim(locale)
+    locales = Gettext.known_locales(GettextBackend)
+
+    cond do
+      locale == "" -> nil
+      # Either it matches directly, eg: "en" => "en", "fr" => "fr"
+      locale in locales -> locale
+      # Either the first part matches, "fr_CA" => "fr"
+      split_locale(locale) in locales -> split_locale(locale)
+      # Otherwise set to default
+      true -> nil
+    end
   end
+
+  def determine_best_locale(_), do: nil
+
+  # Keep only the first part of the locale
+  defp split_locale(locale), do: locale |> String.split("_", trim: true, parts: 2) |> hd
 end
diff --git a/lib/web/router.ex b/lib/web/router.ex
index 915ac241e..bc3a5619e 100644
--- a/lib/web/router.ex
+++ b/lib/web/router.ex
@@ -7,10 +7,12 @@ defmodule Mobilizon.Web.Router do
   pipeline :graphql do
     #    plug(:accepts, ["json"])
     plug(Mobilizon.Web.Auth.Pipeline)
+    plug(Mobilizon.Web.Plugs.SetLocalePlug)
   end
 
   pipeline :graphiql do
     plug(Mobilizon.Web.Auth.Pipeline)
+    plug(Mobilizon.Web.Plugs.SetLocalePlug)
 
     plug(Mobilizon.Web.Plugs.HTTPSecurityPlug,
       script_src: ["cdn.jsdelivr.net"],
@@ -46,6 +48,8 @@ defmodule Mobilizon.Web.Router do
     plug(:accepts, ["html", "activity-json"])
     plug(:put_secure_browser_headers)
 
+    plug(Mobilizon.Web.Plugs.SetLocalePlug)
+
     plug(Cldr.Plug.AcceptLanguage,
       cldr_backend: Mobilizon.Cldr,
       no_match_log_level: :debug
@@ -60,6 +64,8 @@ defmodule Mobilizon.Web.Router do
   pipeline :browser do
     plug(Plug.Static, at: "/", from: "priv/static")
 
+    plug(Mobilizon.Web.Plugs.SetLocalePlug)
+
     plug(Cldr.Plug.AcceptLanguage,
       cldr_backend: Mobilizon.Cldr,
       no_match_log_level: :debug
diff --git a/lib/web/views/changeset_view.ex b/lib/web/views/changeset_view.ex
deleted file mode 100644
index 0fa250a6e..000000000
--- a/lib/web/views/changeset_view.ex
+++ /dev/null
@@ -1,22 +0,0 @@
-defmodule Mobilizon.Web.ChangesetView do
-  @moduledoc """
-  View for changesets in case of errors
-  """
-  use Mobilizon.Web, :view
-
-  @doc """
-  Traverses and translates changeset errors.
-
-  See `Ecto.Changeset.traverse_errors/2` and
-  `Mobilizon.Web.ErrorHelpers.translate_error/1` for more details.
-  """
-  def translate_errors(changeset) do
-    Ecto.Changeset.traverse_errors(changeset, &translate_error/1)
-  end
-
-  def render("error.json", %{changeset: changeset}) do
-    # When encoded, the changeset returns its errors
-    # as a JSON object. So we just pass it forward.
-    %{errors: translate_errors(changeset)}
-  end
-end
diff --git a/lib/web/views/error_helpers.ex b/lib/web/views/error_helpers.ex
index be5a36c7f..5a42c31c2 100644
--- a/lib/web/views/error_helpers.ex
+++ b/lib/web/views/error_helpers.ex
@@ -5,15 +5,6 @@ defmodule Mobilizon.Web.ErrorHelpers do
 
   use Phoenix.HTML
 
-  @doc """
-  Generates tag for inlined form input errors.
-  """
-  def error_tag(form, field) do
-    Enum.map(Keyword.get_values(form.errors, field), fn error ->
-      content_tag(:span, translate_error(error), class: "help-block")
-    end)
-  end
-
   @doc """
   Translates an error message using gettext.
   """
@@ -31,6 +22,7 @@ defmodule Mobilizon.Web.ErrorHelpers do
     #     dngettext "errors", "1 file", "%{count} files", count
     #     dgettext "errors", "is invalid"
     #
+
     if count = opts[:count] do
       Gettext.dngettext(Mobilizon.Web.Gettext, "errors", msg, msg, count, opts)
     else
diff --git a/lib/web/views/utils.ex b/lib/web/views/utils.ex
index 28d167edf..ecc1018c4 100644
--- a/lib/web/views/utils.ex
+++ b/lib/web/views/utils.ex
@@ -53,27 +53,6 @@ defmodule Mobilizon.Web.Views.Utils do
 
   @spec get_locale(Plug.Conn.t()) :: String.t()
   def get_locale(%Plug.Conn{assigns: assigns}) do
-    assigns
-    |> Map.get(:locale)
-    |> check_locale()
-  end
-
-  def get_locale(_), do: default_locale()
-
-  defp check_locale(nil) do
-    default_locale()
-    |> check_locale()
-  end
-
-  defp check_locale("") do
-    check_locale(nil)
-  end
-
-  defp check_locale(locale) when is_binary(locale), do: locale
-
-  defp default_locale do
-    Mobilizon.Config.instance_config()
-    |> Keyword.get(:default_language, "en")
-    |> Kernel.||("en")
+    Map.get(assigns, :locale)
   end
 end
diff --git a/mix.exs b/mix.exs
index be869d327..866e157c3 100644
--- a/mix.exs
+++ b/mix.exs
@@ -300,7 +300,6 @@ defmodule Mobilizon.Mixfile do
         Mobilizon.Web.ChangesetView,
         Mobilizon.Web.JsonLD.ObjectView,
         Mobilizon.Web.EmailView,
-        Mobilizon.Web.ErrorHelpers,
         Mobilizon.Web.ErrorView,
         Mobilizon.Web.LayoutView,
         Mobilizon.Web.PageView,
diff --git a/test/graphql/resolvers/event_test.exs b/test/graphql/resolvers/event_test.exs
index ec07b5bd9..d2dc8c5ff 100644
--- a/test/graphql/resolvers/event_test.exs
+++ b/test/graphql/resolvers/event_test.exs
@@ -374,7 +374,7 @@ defmodule Mobilizon.Web.Resolvers.EventTest do
         )
 
       assert hd(res["errors"])["message"] == %{
-               "maximum_attendee_capacity" => ["must be greater than or equal to %{number}"]
+               "maximum_attendee_capacity" => ["must be greater than or equal to 0"]
              }
     end
 
diff --git a/test/graphql/resolvers/user_test.exs b/test/graphql/resolvers/user_test.exs
index 1b8f3d690..617bf9094 100644
--- a/test/graphql/resolvers/user_test.exs
+++ b/test/graphql/resolvers/user_test.exs
@@ -308,6 +308,7 @@ defmodule Mobilizon.GraphQL.Resolvers.UserTest do
     test "create_user/3 doesn't allow two users with the same email", %{conn: conn} do
       res =
         conn
+        |> put_req_header("accept-language", "fr")
         |> AbsintheHelpers.graphql_query(
           query: @create_user_mutation,
           variables: @user_creation
@@ -397,6 +398,7 @@ defmodule Mobilizon.GraphQL.Resolvers.UserTest do
 
     test "register_person/3 doesn't register a profile from an unknown email", %{conn: conn} do
       conn
+      |> put_req_header("accept-language", "fr")
       |> AbsintheHelpers.graphql_query(
         query: @create_user_mutation,
         variables: @user_creation
@@ -416,6 +418,7 @@ defmodule Mobilizon.GraphQL.Resolvers.UserTest do
 
     test "register_person/3 can't be called with an existing profile", %{conn: conn} do
       conn
+      |> put_req_header("accept-language", "fr")
       |> AbsintheHelpers.graphql_query(
         query: @create_user_mutation,
         variables: @user_creation
@@ -423,6 +426,7 @@ defmodule Mobilizon.GraphQL.Resolvers.UserTest do
 
       res =
         conn
+        |> put_req_header("accept-language", "fr")
         |> AbsintheHelpers.graphql_query(
           query: @register_person_mutation,
           variables: @user_creation
@@ -447,6 +451,7 @@ defmodule Mobilizon.GraphQL.Resolvers.UserTest do
       insert(:actor, preferred_username: "myactor")
 
       conn
+      |> put_req_header("accept-language", "fr")
       |> AbsintheHelpers.graphql_query(
         query: @create_user_mutation,
         variables: @user_creation
diff --git a/test/web/gettext_test.exs b/test/web/gettext_test.exs
deleted file mode 100644
index 3ed4f9623..000000000
--- a/test/web/gettext_test.exs
+++ /dev/null
@@ -1,39 +0,0 @@
-defmodule Mobilizon.Web.GettextTest do
-  use ExUnit.Case, async: true
-
-  alias Mobilizon.Config
-  alias Mobilizon.Web.Gettext, as: GettextBackend
-
-  describe "test determine_best_locale/1" do
-    setup do
-      Config.put([:instance, :default_language], "en")
-      :ok
-    end
-
-    test "with empty string returns the default locale" do
-      assert GettextBackend.determine_best_locale("") == "en"
-    end
-
-    test "with empty string returns the default configured locale" do
-      Config.put([:instance, :default_language], "es")
-      assert GettextBackend.determine_best_locale("") == "es"
-    end
-
-    test "with empty string returns english as a proper fallback if the default configured locale is nil" do
-      Config.put([:instance, :default_language], nil)
-      assert GettextBackend.determine_best_locale("") == "en"
-    end
-
-    test "returns fallback with an unexisting locale" do
-      assert GettextBackend.determine_best_locale("yolo") == "en"
-    end
-
-    test "maps the correct part if the locale has multiple ones" do
-      assert GettextBackend.determine_best_locale("fr_CA") == "fr"
-    end
-
-    test "returns the locale if valid" do
-      assert GettextBackend.determine_best_locale("es") == "es"
-    end
-  end
-end
diff --git a/test/web/plugs/detect_locale_plug_test.exs b/test/web/plugs/detect_locale_plug_test.exs
new file mode 100644
index 000000000..84d001b98
--- /dev/null
+++ b/test/web/plugs/detect_locale_plug_test.exs
@@ -0,0 +1,35 @@
+# Portions of this file are derived from Pleroma:
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Mobilizon.Web.Plugs.DetectLocalePlugTest do
+  use ExUnit.Case, async: true
+  use Plug.Test
+
+  alias Mobilizon.Web.Plugs.DetectLocalePlug
+  alias Plug.Conn
+
+  test "use supported locale from `accept-language`" do
+    conn =
+      :get
+      |> conn("/cofe")
+      |> Conn.put_req_header(
+        "accept-language",
+        "ru, fr-CH, fr;q=0.9, en;q=0.8, *;q=0.5"
+      )
+      |> DetectLocalePlug.call([])
+
+    assert %{detected_locale: "ru"} == conn.assigns
+  end
+
+  test "returns empty string if `accept-language` header is empty" do
+    conn =
+      :get
+      |> conn("/cofe")
+      |> Conn.put_req_header("accept-language", "tlh")
+      |> DetectLocalePlug.call([])
+
+    assert %{detected_locale: nil} == conn.assigns
+  end
+end
diff --git a/test/web/plugs/set_locale_plug_test.exs b/test/web/plugs/set_locale_plug_test.exs
index 8804a8a54..924765ea3 100644
--- a/test/web/plugs/set_locale_plug_test.exs
+++ b/test/web/plugs/set_locale_plug_test.exs
@@ -7,42 +7,87 @@ defmodule Mobilizon.Web.Plugs.SetLocalePlugTest do
   use ExUnit.Case, async: true
   use Plug.Test
 
-  alias Mobilizon.Web.Gettext, as: GettextBackend
+  alias Mobilizon.Config
   alias Mobilizon.Web.Plugs.SetLocalePlug
   alias Plug.Conn
 
-  test "default locale is `en`" do
-    conn =
-      :get
-      |> conn("/cofe")
-      |> SetLocalePlug.call([])
+  describe "test assigning locale to conn" do
+    test "use supported locale from `accept-language`" do
+      conn =
+        :get
+        |> conn("/cofe")
+        |> assign(:detected_locale, "ru")
+        |> SetLocalePlug.call([])
 
-    assert "en" == Gettext.get_locale()
-    assert %{locale: ""} == conn.assigns
+      assert "ru" == Gettext.get_locale()
+      assert %{locale: "ru", detected_locale: "ru"} == conn.assigns
+    end
+
+    test "use default locale if locale from `accept-language` is not supported" do
+      conn =
+        :get
+        |> conn("/cofe")
+        |> Conn.put_req_header("accept-language", "tlh")
+        |> SetLocalePlug.call([])
+
+      assert "en" == Gettext.get_locale()
+      assert %{locale: "en"} == conn.assigns
+    end
   end
 
-  test "use supported locale from `accept-language`" do
-    conn =
-      :get
-      |> conn("/cofe")
-      |> Conn.put_req_header(
-        "accept-language",
-        "ru, fr-CH, fr;q=0.9, en;q=0.8, *;q=0.5"
-      )
-      |> SetLocalePlug.call([])
+  describe "test getting default locale from instance" do
+    test "default locale is `en`" do
+      conn =
+        :get
+        |> conn("/cofe")
+        |> SetLocalePlug.call([])
 
-    assert "ru" == Gettext.get_locale(GettextBackend)
-    assert %{locale: "ru"} == conn.assigns
+      assert "en" == Gettext.get_locale()
+      assert %{locale: "en"} == conn.assigns
+    end
+
+    test "with empty string returns the default configured locale" do
+      Config.put([:instance, :default_language], "es")
+
+      conn =
+        :get
+        |> conn("/cofe")
+        |> SetLocalePlug.call([])
+
+      assert %{locale: "es"} == conn.assigns
+
+      Config.put([:instance, :default_language], "en")
+    end
+
+    test "with empty string returns english as a proper fallback if the default configured locale is nil" do
+      Config.put([:instance, :default_language], nil)
+
+      conn =
+        :get
+        |> conn("/cofe")
+        |> SetLocalePlug.call([])
+
+      assert %{locale: "en"} == conn.assigns
+
+      Config.put([:instance, :default_language], "en")
+    end
   end
 
-  test "use default locale if locale from `accept-language` is not supported" do
-    conn =
-      :get
-      |> conn("/cofe")
-      |> Conn.put_req_header("accept-language", "tlh")
-      |> SetLocalePlug.call([])
+  describe "test determine_best_locale/1" do
+    test "with empty string returns the default locale" do
+      assert SetLocalePlug.determine_best_locale("") == nil
+    end
 
-    assert "en" == Gettext.get_locale(GettextBackend)
-    assert %{locale: ""} == conn.assigns
+    test "returns fallback with an unexisting locale" do
+      assert SetLocalePlug.determine_best_locale("yolo") == nil
+    end
+
+    test "maps the correct part if the locale has multiple ones" do
+      assert SetLocalePlug.determine_best_locale("fr_CA") == "fr"
+    end
+
+    test "returns the locale if valid" do
+      assert SetLocalePlug.determine_best_locale("es") == "es"
+    end
   end
 end