diff --git a/package-lock.json b/package-lock.json
index 1775821b8..96b62316d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -4990,7 +4990,8 @@
     "es6-promise": {
       "version": "4.2.8",
       "resolved": "https://registry.npmjs.org/es6-promise/-/es6-promise-4.2.8.tgz",
-      "integrity": "sha512-HJDGx5daxeIvxdBxvG2cb9g4tEvwIk3i8+nhX0yGrYmZUzbkdg8QbDevheDB8gd0//uPj4c1EQua8Q+MViT0/w=="
+      "integrity": "sha512-HJDGx5daxeIvxdBxvG2cb9g4tEvwIk3i8+nhX0yGrYmZUzbkdg8QbDevheDB8gd0//uPj4c1EQua8Q+MViT0/w==",
+      "dev": true
     },
     "es6-promisify": {
       "version": "5.0.0",
diff --git a/src/converse-chatview.js b/src/converse-chatview.js
index c5fc5196a..97cc55c61 100644
--- a/src/converse-chatview.js
+++ b/src/converse-chatview.js
@@ -29,7 +29,7 @@ import tpl_toolbar_fileupload from "templates/toolbar_fileupload.html";
 import tpl_user_details_modal from "templates/user_details_modal.html";
 import xss from "xss/dist/xss";
 
-const { $msg, Backbone, Promise, Strophe, _, sizzle, dayjs } = converse.env;
+const { $msg, Backbone, Strophe, _, sizzle, dayjs } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/converse-controlbox.js b/src/converse-controlbox.js
index ff28874f9..ce2fbc1f4 100644
--- a/src/converse-controlbox.js
+++ b/src/converse-controlbox.js
@@ -17,7 +17,7 @@ import tpl_controlbox_toggle from "templates/controlbox_toggle.html";
 import tpl_login_panel from "templates/login_panel.html";
 
 const CHATBOX_TYPE = 'chatbox';
-const { Strophe, Backbone, Promise, _, dayjs } = converse.env;
+const { Strophe, Backbone, _, dayjs } = converse.env;
 const u = converse.env.utils;
 
 const CONNECTION_STATUS_CSS_CLASS = {
diff --git a/src/converse-minimize.js b/src/converse-minimize.js
index c4a3d0687..f3e65f3de 100644
--- a/src/converse-minimize.js
+++ b/src/converse-minimize.js
@@ -16,7 +16,7 @@ import tpl_toggle_chats from "templates/toggle_chats.html";
 import tpl_trimmed_chat from "templates/trimmed_chat.html";
 
 
-const { _ , Backbone, Promise, Strophe, dayjs } = converse.env;
+const { _ , Backbone, Strophe, dayjs } = converse.env;
 const u = converse.env.utils;
 
 converse.plugins.add('converse-minimize', {
diff --git a/src/converse-muc-views.js b/src/converse-muc-views.js
index 2a525246a..6273fddc3 100644
--- a/src/converse-muc-views.js
+++ b/src/converse-muc-views.js
@@ -43,7 +43,7 @@ import tpl_spinner from "templates/spinner.html";
 import xss from "xss/dist/xss";
 
 
-const { Backbone, Promise, Strophe, dayjs, sizzle, _, $iq, $msg, $pres } = converse.env;
+const { Backbone, Strophe, dayjs, sizzle, _, $iq, $msg, $pres } = converse.env;
 const u = converse.env.utils;
 
 const ROLES = ['moderator', 'participant', 'visitor'];
diff --git a/src/converse-omemo.js b/src/converse-omemo.js
index 8f79cdc11..044f7ab02 100644
--- a/src/converse-omemo.js
+++ b/src/converse-omemo.js
@@ -13,7 +13,7 @@ import BrowserStorage from "backbone.browserStorage";
 import converse from "@converse/headless/converse-core";
 import tpl_toolbar_omemo from "templates/toolbar_omemo.html";
 
-const { Backbone, Promise, Strophe, dayjs, sizzle, $build, $iq, $msg, _ } = converse.env;
+const { Backbone, Strophe, dayjs, sizzle, $build, $iq, $msg, _ } = converse.env;
 const u = converse.env.utils;
 
 Strophe.addNamespace('OMEMO_DEVICELIST', Strophe.NS.OMEMO+".devicelist");
diff --git a/src/converse-profile.js b/src/converse-profile.js
index 140a344b3..96c655c79 100644
--- a/src/converse-profile.js
+++ b/src/converse-profile.js
@@ -18,8 +18,7 @@ import tpl_profile_modal from "templates/profile_modal.html";
 import tpl_profile_view from "templates/profile_view.html";
 import tpl_status_option from "templates/status_option.html";
 
-
-const { Strophe, Backbone, Promise, utils, _, dayjs, sizzle } = converse.env;
+const { Strophe, Backbone, utils, _, dayjs, sizzle } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/converse-roomslist.js b/src/converse-roomslist.js
index 7090dffa1..e5bd6ba03 100644
--- a/src/converse-roomslist.js
+++ b/src/converse-roomslist.js
@@ -16,7 +16,7 @@ import muc from "@converse/headless/converse-muc";
 import tpl_rooms_list from "templates/rooms_list.html";
 import tpl_rooms_list_item from "templates/rooms_list_item.html"
 
-const { Backbone, Promise, Strophe, sizzle, _ } = converse.env;
+const { Backbone, Strophe, sizzle, _ } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/headless/converse-bookmarks.js b/src/headless/converse-bookmarks.js
index d4708d01d..22c109ac4 100644
--- a/src/headless/converse-bookmarks.js
+++ b/src/headless/converse-bookmarks.js
@@ -13,7 +13,7 @@ import "@converse/headless/converse-muc";
 import BrowserStorage from "backbone.browserStorage";
 import converse from "@converse/headless/converse-core";
 
-const { Backbone, Promise, Strophe, $iq, sizzle, _ } = converse.env;
+const { Backbone, Strophe, $iq, sizzle, _ } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/headless/converse-chatboxes.js b/src/headless/converse-chatboxes.js
index 8723ab0a2..147e18246 100644
--- a/src/headless/converse-chatboxes.js
+++ b/src/headless/converse-chatboxes.js
@@ -12,7 +12,7 @@ import BrowserStorage from "backbone.browserStorage";
 import converse from "./converse-core";
 import filesize from "filesize";
 
-const { $msg, Backbone, Promise, Strophe, dayjs, sizzle, utils, _ } = converse.env;
+const { $msg, Backbone, Strophe, dayjs, sizzle, utils, _ } = converse.env;
 const u = converse.env.utils;
 
 Strophe.addNamespace('MESSAGE_CORRECT', 'urn:xmpp:message-correct:0');
diff --git a/src/headless/converse-core.js b/src/headless/converse-core.js
index 5c3318f23..f5e63001c 100644
--- a/src/headless/converse-core.js
+++ b/src/headless/converse-core.js
@@ -11,7 +11,6 @@ import 'strophe.js/src/websocket';
 import * as strophe from 'strophe.js/src/core';
 import Backbone from 'backbone';
 import BrowserStorage from 'backbone.browserStorage';
-import Promise from 'es6-promise/dist/es6-promise.auto';
 import _ from './lodash.noconflict';
 import advancedFormat from 'dayjs/plugin/advancedFormat';
 import dayjs from 'dayjs';
@@ -832,10 +831,15 @@ _converse.initialize = async function (settings, callback) {
     });
 
     /* Localisation */
-    if (i18n !== undefined) {
-        i18n.setLocales(settings.i18n, _converse);
-    } else {
+    if (i18n === undefined || _converse.isTestEnv()) {
         _converse.locale = 'en';
+    } else {
+        try {
+            _converse.locale = i18n.getLocale(settings.i18n, _converse.locales);
+            await i18n.fetchTranslations(_converse);
+        } catch (e) {
+            _converse.log(e.message, Strophe.LogLevel.FATAL);
+        }
     }
 
     // Module-level variables
@@ -1331,18 +1335,12 @@ _converse.initialize = async function (settings, callback) {
         this.connection = settings.connection;
     }
 
-    if (_converse.isTestEnv()) {
-        await finishInitialization();
-        return _converse;
-    } else if (i18n !== undefined) {
-        try {
-            await i18n.fetchTranslations(_converse.locale, _converse.locales);
-        } catch (e) {
-            _converse.log(e.message, Strophe.LogLevel.FATAL);
-        }
-    }
     await finishInitialization();
-    return init_promise;
+    if (_converse.isTestEnv()) {
+        return _converse;
+    } else {
+        return init_promise;
+    }
 };
 
 /**
diff --git a/src/headless/converse-disco.js b/src/headless/converse-disco.js
index bb85e3837..9d325e42e 100644
--- a/src/headless/converse-disco.js
+++ b/src/headless/converse-disco.js
@@ -12,7 +12,7 @@ import BrowserStorage from "backbone.browserStorage";
 import converse from "./converse-core";
 import sizzle from "sizzle";
 
-const { Backbone, Promise, Strophe, $iq, utils, _ } = converse.env;
+const { Backbone, Strophe, $iq, utils, _ } = converse.env;
 
 converse.plugins.add('converse-disco', {
 
diff --git a/src/headless/converse-muc.js b/src/headless/converse-muc.js
index 7c9bd438f..560db50a9 100644
--- a/src/headless/converse-muc.js
+++ b/src/headless/converse-muc.js
@@ -23,7 +23,7 @@ const MUC_ROLE_WEIGHTS = {
     'none':         2,
 };
 
-const { Strophe, Backbone, Promise, $iq, $build, $msg, $pres, sizzle, _ } = converse.env;
+const { Strophe, Backbone, $iq, $build, $msg, $pres, sizzle, _ } = converse.env;
 
 // Add Strophe Namespaces
 Strophe.addNamespace('MUC_ADMIN', Strophe.NS.MUC + "#admin");
diff --git a/src/headless/converse-pubsub.js b/src/headless/converse-pubsub.js
index e8c88a4ca..d695c5747 100644
--- a/src/headless/converse-pubsub.js
+++ b/src/headless/converse-pubsub.js
@@ -9,7 +9,7 @@
 import "./converse-disco";
 import converse from "./converse-core";
 
-const { Strophe, Backbone, Promise, $iq, $build, $msg, $pres, dayjs, _ } = converse.env;
+const { Strophe, Backbone, $iq, $build, $msg, $pres, dayjs, _ } = converse.env;
 
 Strophe.addNamespace('PUBSUB_ERROR', Strophe.NS.PUBSUB+"#errors");
 
diff --git a/src/headless/converse-roster.js b/src/headless/converse-roster.js
index e2a995b51..05fbcdf08 100644
--- a/src/headless/converse-roster.js
+++ b/src/headless/converse-roster.js
@@ -9,7 +9,7 @@
 import BrowserStorage from "backbone.browserStorage";
 import converse from "@converse/headless/converse-core";
 
-const { Backbone, Promise, Strophe, $iq, $pres, dayjs, sizzle, _ } = converse.env;
+const { Backbone, Strophe, $iq, $pres, dayjs, sizzle, _ } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/headless/converse-vcard.js b/src/headless/converse-vcard.js
index 1c58c809b..5662183d2 100644
--- a/src/headless/converse-vcard.js
+++ b/src/headless/converse-vcard.js
@@ -10,7 +10,7 @@ import BrowserStorage from "backbone.browserStorage";
 import converse from "./converse-core";
 import tpl_vcard from "./templates/vcard.html";
 
-const { Backbone, Promise, Strophe, _, $iq, $build, dayjs, sizzle } = converse.env;
+const { Backbone, Strophe, _, $iq, $build, dayjs, sizzle } = converse.env;
 const u = converse.env.utils;
 
 
diff --git a/src/headless/i18n.js b/src/headless/i18n.js
index 00fc0f76a..5ec3f20a9 100644
--- a/src/headless/i18n.js
+++ b/src/headless/i18n.js
@@ -6,40 +6,12 @@
 // Copyright (c) 2013-2017, Jan-Carel Brand <jc@opkode.com>
 // Licensed under the Mozilla Public License (MPLv2)
 //
-import 'dayjs/locale/af';
-import 'dayjs/locale/ar';
-import 'dayjs/locale/bg';
-import 'dayjs/locale/ca';
-import 'dayjs/locale/cs';
-import 'dayjs/locale/de';
-import 'dayjs/locale/eo';
-import 'dayjs/locale/es';
-import 'dayjs/locale/eu';
-import 'dayjs/locale/fr';
-import 'dayjs/locale/gl';
-import 'dayjs/locale/he';
-import 'dayjs/locale/hi';
-import 'dayjs/locale/hu';
-import 'dayjs/locale/id';
-import 'dayjs/locale/it';
-import 'dayjs/locale/ja';
-import 'dayjs/locale/nb';
-import 'dayjs/locale/nl';
-import 'dayjs/locale/oc-lnc';
-import 'dayjs/locale/pl';
-import 'dayjs/locale/pt';
-import 'dayjs/locale/pt-br';
-import 'dayjs/locale/ro';
-import 'dayjs/locale/ru';
-import 'dayjs/locale/tr';
-import 'dayjs/locale/uk';
-import 'dayjs/locale/zh-cn';
-import 'dayjs/locale/zh-tw';
+import * as strophe from 'strophe.js/src/core';
 import Jed from "jed";
-import Promise from "es6-promise/dist/es6-promise.auto";
-import _ from "./lodash.noconflict";
 import dayjs from "dayjs";
 
+const Strophe = strophe.default.Strophe;
+
 
 function detectLocale (library_check) {
     /* Determine which locale is supported by the user's system as well
@@ -69,11 +41,11 @@ function detectLocale (library_check) {
 }
 
 function isConverseLocale (locale, supported_locales) {
-    return _.isString(locale) && _.includes(supported_locales, locale);
+    return typeof locale === 'string' && supported_locales.includes(locale);
 }
 
 function getLocale (preferred_locale, isSupportedByLibrary) {
-    if (_.isString(preferred_locale)) {
+    if (typeof preferred_locale === 'string') {
         if (preferred_locale === 'en' || isSupportedByLibrary(preferred_locale)) {
             return preferred_locale;
         }
@@ -103,12 +75,8 @@ let jed_instance;
  */
 export default {
 
-    setLocales (preferred_locale, _converse) {
-        _converse.locale = getLocale(
-            preferred_locale,
-            _.partial(isConverseLocale, _, _converse.locales)
-        );
-        dayjs.locale(getLocale(preferred_locale, l => dayjs.locale(l)));
+    getLocale (preferred_locale, available_locales) {
+        return getLocale(preferred_locale, preferred => isConverseLocale(preferred, available_locales));
     },
 
     translate (str) {
@@ -127,14 +95,16 @@ export default {
      * Fetch the translations for the given local at the given URL.
      * @private
      * @method i18n#fetchTranslations
-     * @param { String } locale -The given i18n locale
-     * @param { Array } supported_locales -  List of locales supported
+     * @param { _converse }
      */
-    async fetchTranslations (locale, supported_locales) {
-        if (!isConverseLocale(locale, supported_locales) || locale === 'en') {
+    async fetchTranslations (_converse) {
+        const locale = _converse.locale;
+        if (!isConverseLocale(locale, _converse.locales) || locale === 'en') {
             return;
         }
         const { default: data } = await import(/*webpackChunkName: "locales/[request]" */ `../../locale/${locale}/LC_MESSAGES/converse.po`);
+        await import(/*webpackChunkName: "locales/dayjs/[request]" */ `dayjs/locale/${locale.toLowerCase().replace('_', '-')}`);
+        dayjs.locale(getLocale(_converse.locale, l => dayjs.locale(l)));
         jed_instance = new Jed(data);
     }
 };
diff --git a/src/headless/package.json b/src/headless/package.json
index 02e7b3c44..531d8e9c3 100644
--- a/src/headless/package.json
+++ b/src/headless/package.json
@@ -24,7 +24,6 @@
   "devDependencies": {
     "backbone": "1.4",
     "backbone.browserStorage": "0.0.5",
-    "es6-promise": "^4.2.8",
     "filesize": "^4.1.2",
     "jed": "1.1.1",
     "lodash": "^4.17.14",