Locale is added to all sent emails, as a header

See https://tools.ietf.org/html/rfc2076#section-3.10

app/mailers/event_mailer.rb

@@ -8,7 +8,8 @@ class EventMailer < ApplicationMailer
     mail 'Message-ID' =>
       "<event-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: event.submitter,
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   # Send email to submitter too. Before, only moderators were receiving emails
@@ -20,7 +21,8 @@ class EventMailer < ApplicationMailer
     mail 'In-Reply-To' =>
       "<event-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: event.submitter,
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   def accept(event)
@@ -31,7 +33,8 @@ class EventMailer < ApplicationMailer
       "<event-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: event.submitter,
          subject: "#{t 'mail_prefix'}#{t 'event_mailer.accept.subject',
-                                         subject: event.title}"
+                                         subject: event.title}",
+         'Content-Language': locale
   end
 
   def destroy(event)
@@ -42,7 +45,8 @@ class EventMailer < ApplicationMailer
       "<event-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: event.submitter,
          subject: "#{t 'mail_prefix'}#{t 'event_mailer.destroy.subject',
-                                         subject: event.title}"
+                                         subject: event.title}",
+         'Content-Language': locale
   end
 
   private

app/mailers/moderation_mailer.rb

@@ -7,7 +7,8 @@ class ModerationMailer < ApplicationMailer
 
     mail 'Message-ID' =>
       "<mod-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   def update(event)
@@ -16,7 +17,8 @@ class ModerationMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<mod-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   def accept(event)
@@ -25,7 +27,8 @@ class ModerationMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<mod-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   def destroy(event)
@@ -34,7 +37,8 @@ class ModerationMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<mod-#{event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(event)
+         subject: subject(event),
+         'Content-Language': locale
   end
 
   private

app/mailers/moderationorga_mailer.rb

@@ -7,7 +7,8 @@ class ModerationorgaMailer < ApplicationMailer
 
     mail 'Message-ID' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def update(orga)
@@ -16,7 +17,8 @@ class ModerationorgaMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def accept(orga)
@@ -25,7 +27,8 @@ class ModerationorgaMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def destroy(orga, reason = '')
@@ -35,7 +38,8 @@ class ModerationorgaMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   private

app/mailers/note_mailer.rb

@@ -9,7 +9,8 @@ class NoteMailer < ApplicationMailer
     mail 'In-Reply-To' =>
       "<event-#{note.event.id}@#{host}>",
          to: note.event.submitter,
-         subject: subject(note.event)
+         subject: subject(note.event),
+         'Content-Language': locale
   end
 
   def create(note)
@@ -17,7 +18,8 @@ class NoteMailer < ApplicationMailer
 
     mail 'In-Reply-To' =>
       "<mod-#{note.event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
-         subject: subject(note.event)
+         subject: subject(note.event),
+         'Content-Language': locale
   end
 
   private

app/mailers/orga_mailer.rb

@@ -8,7 +8,8 @@ class OrgaMailer < ApplicationMailer
     mail 'Message-ID' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: orga.submitter,
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def update(orga)
@@ -18,7 +19,8 @@ class OrgaMailer < ApplicationMailer
     mail 'Message-ID' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: orga.submitter,
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def accept(orga)
@@ -28,7 +30,8 @@ class OrgaMailer < ApplicationMailer
     mail 'In-Reply-To' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: orga.submitter,
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   def destroy(orga, reason = '')
@@ -39,7 +42,8 @@ class OrgaMailer < ApplicationMailer
     mail 'In-Reply-To' =>
       "<orga-#{orga.id}@#{ActionMailer::Base.default_url_options[:host]}>",
          to: orga.submitter,
-         subject: subject(orga)
+         subject: subject(orga),
+         'Content-Language': locale
   end
 
   private