diff --git a/build/connect.js b/build/connect.js
index b117d6c8e10..830be675402 100644
--- a/build/connect.js
+++ b/build/connect.js
@@ -45,6 +45,16 @@ module.exports = function (grunt) {
                     middlewares.push(serveIndex(directory));
 
                     middlewares.push((req, res) => {
+                        const pathname = req.url.split('?')[0];
+                        const pattern = `/(?!${lang_regex})\\w{2,5}/`;
+                        if (new RegExp(`^${pattern}\\w+`).test(pathname)) {
+                            const en_pathname  = pathname.replace(new RegExp(pattern), '/en/');
+                            const en_file_path = `${options.base[0]}${en_pathname}`;
+                            if (grunt.file.exists(en_file_path)) {
+                                require('fs').createReadStream(en_file_path).pipe(res);
+                                return;
+                            }
+                        }
                         const path_404 = `${options.base[0]}/404.html`;
                         if (grunt.file.exists(path_404)) {
                             require('fs').createReadStream(path_404).pipe(res);
diff --git a/src/javascript/_common/language.js b/src/javascript/_common/language.js
index aab72e7c05f..402fc0970e1 100644
--- a/src/javascript/_common/language.js
+++ b/src/javascript/_common/language.js
@@ -37,7 +37,10 @@ const Language = (() => {
     const languageFromUrl = (custom_url) => {
         if (url_lang && !custom_url) return url_lang;
         const url_params = (custom_url || window.location.href).split('/').slice(3);
-        const language   = (url_params.find(lang => lang_regex.test(lang)) || '');
+        let language     = (url_params.find(lang => lang_regex.test(lang)) || '');
+        if (language && !Object.keys(all_languages).includes(language.toUpperCase())) {
+            language = 'en';
+        }
         if (!custom_url) {
             url_lang = language;
         }
@@ -57,6 +60,9 @@ const Language = (() => {
                 }
             }
         }
+        if (current_lang && languageFromUrl() && current_lang !== languageFromUrl()) {
+            current_lang = languageFromUrl();
+        }
         current_lang = (current_lang || (languageFromUrl() || Cookies.get('language') || default_language).toUpperCase());
         return current_lang;
     };
diff --git a/src/javascript/app/base/binary_loader.js b/src/javascript/app/base/binary_loader.js
index 63eb3411840..d689bc6c9c3 100644
--- a/src/javascript/app/base/binary_loader.js
+++ b/src/javascript/app/base/binary_loader.js
@@ -11,6 +11,7 @@ const GTM = require('../../_common/base/gtm');
 const Login = require('../../_common/base/login');
 const LiveChat = require('../../_common/base/livechat');
 const getElementById = require('../../_common/common_functions').getElementById;
+const getAll = require('../../_common/language').getAll;
 const urlLang = require('../../_common/language').urlLang;
 const localizeForLang = require('../../_common/localize').forLang;
 const localize = require('../../_common/localize').localize;
@@ -25,6 +26,16 @@ const BinaryLoader = (() => {
     let active_script = null;
 
     const init = () => {
+        const supported_langs_regex = Object.keys(getAll()).map(lang => lang.toLowerCase()).join('|');
+        const pathname = window.location.pathname;
+        const search = window.location.search;
+        const pattern = `/(?!${supported_langs_regex})\\w{2,5}/`;
+        // redirect to /en/ page if pathname contains an unsupported language:
+        if (new RegExp(`^${pattern}\\w+`).test(pathname)) {
+            const en_pathname  = pathname.replace(new RegExp(pattern), '/en/');
+            const en_href = `${window.location.origin}${en_pathname}${search || ''}`;
+            window.history.replaceState({ url: en_href }, document.title, en_href);
+        }
 
         if (!isStorageSupported(localStorage) || !isStorageSupported(sessionStorage)) {
             Header.displayNotification({ key: 'storage_not_supported', title: 'Storage not supported', message: localize('Requires your browser\'s web storage to be enabled in order to function properly. Please enable it or exit private browsing mode.'), type: 'danger' });
diff --git a/src/root_files/_common/404.html b/src/root_files/_common/404.html
index e02881f591d..23e90ca83f6 100644
--- a/src/root_files/_common/404.html
+++ b/src/root_files/_common/404.html
@@ -11,18 +11,17 @@
     }
 
     var langs = {
-        // TODO: uncomment if we enable de again
-        // 'de': 'de|at|li',
+        // TODO: uncomment if we enable id, ko and pt languages
+        // 'id': 'id',
+        // 'ko': 'kr',
+        // 'pt': 'br|mz|ao|pt|gw|pg|cv|st',
+        'de': 'de|at|li',
         'es': 'ar|bo|cl|co|cr|cu|do|ec|sv|gt|hn|mx|ni|pa|py|pr|es|uy|ve',
         'fr': 'fr|ad|bj|bf|cf|cg|ga|gn|ml|mc|ne|sn|tg',
-        'id': 'id',
         'it': 'it',
-        'ko': 'kr',
         'pl': 'po',
-        'pt': 'br|mz|ao|pt|gw|pg|cv|st',
         'ru': 'ru|ua|by|kz',
-        // TODO: uncomment if we enable th again
-        // 'th': 'th',
+        'th': 'th',
         'vi': 'vn',
         'zh_tw': 'tw|mo',
         'zh_cn': 'cn',
@@ -44,11 +43,11 @@
     function redirect(lang) {
         var base = /^(.+?).github.io$/.test(window.location.hostname) ? window.location.origin + '/binary-static' : window.location.origin;
 
-        // TODO: REMOVE/CHANGE this after addition of DE or TH languages
-        var unsupported_languages = ['de', 'th'];
-        if (unsupported_languages.indexOf(lang.toLowerCase()) >= 0) {
+        // redirect to /en/ page if pathname contains an unsupported language:
+        const supported_langs_regex = Object.keys(langs).join('|');
+        if (new RegExp(`^/(?!${supported_langs_regex})\\w{2,5}/\\w+`).test(window.location.pathname.toLowerCase())) {
             lang = 'en';
-            window.location.href = base + '/' + lang.toLowerCase() + '/' + 'trading.html';
+            window.location.href = base + '/' + lang + '/' + 'trading.html';
             return;
         }