diff --git a/libretranslate/app.py b/libretranslate/app.py
index 24ec170..e877579 100644
--- a/libretranslate/app.py
+++ b/libretranslate/app.py
@@ -9,9 +9,10 @@ from timeit import default_timer
 import argostranslatefiles
 from argostranslatefiles import get_supported_formats
 from flask import (abort, Blueprint, Flask, jsonify, render_template, request,
-                   Response, send_file, url_for)
+                   Response, send_file, url_for, session)
 from flask_swagger import swagger
 from flask_swagger_ui import get_swaggerui_blueprint
+from flask_session import Session
 from translatehtml import translate_html
 from werkzeug.utils import secure_filename
 from werkzeug.exceptions import HTTPException
@@ -277,6 +278,10 @@ def create_app(args):
         if args.disable_web_ui:
             abort(404)
 
+        langcode = request.args.get('lang')
+        if langcode and langcode in get_available_locale_codes(not args.debug):
+            session.update(preferred_lang=langcode)
+
         return render_template(
             "index.html",
             gaId=args.ga_id,
@@ -286,19 +291,19 @@ def create_app(args):
             web_version=os.environ.get("LT_WEB") is not None,
             version=get_version(),
             swagger_url=SWAGGER_URL,
-            url_prefix=args.url_prefix,
-            available_locales=[{'code': l['code'], 'name': _lazy(l['name'])} for l in get_available_locales()],
+            available_locales=[{'code': l['code'], 'name': _lazy(l['name'])} for l in get_available_locales(not args.debug)],
             current_locale=get_locale(),
             alternate_locales=get_alternate_locale_links()
         )
 
-    @bp.route("/static/js/app.js")
+    @bp.route("/js/app.js")
     @limiter.exempt
     def appjs():
       if args.disable_web_ui:
             abort(404)
 
       return render_template("app.js.template", 
+            url_prefix=args.url_prefix,
             get_api_key_link=args.get_api_key_link)
 
     @bp.get("/languages")
@@ -991,6 +996,11 @@ def create_app(args):
         return jsonify({"success": True})
 
     app = Flask(__name__)
+
+    app.config["SESSION_TYPE"] = "filesystem"
+    app.config["SESSION_FILE_DIR"] = os.path.join("db", "sessions")
+    Session(app)
+
     if args.debug:
         app.config["TEMPLATES_AUTO_RELOAD"] = True
     if args.url_prefix:
@@ -1009,12 +1019,14 @@ def create_app(args):
     def spec():
         return jsonify(lazy_swag(swag))
 
-
     app.config["BABEL_TRANSLATION_DIRECTORIES"] = 'locales'
     babel = Babel(app)
     @babel.localeselector
     def get_locale():
-        return request.accept_languages.best_match(get_available_locale_codes())
+        override_lang = request.headers.get('X-Override-Accept-Language')
+        if override_lang and override_lang in get_available_locale_codes():
+            return override_lang
+        return session.get('preferred_lang', request.accept_languages.best_match(get_available_locale_codes()))
 
     app.jinja_env.globals.update(_e=gettext_escaped, _h=gettext_html)
 
diff --git a/libretranslate/templates/app.js.template b/libretranslate/templates/app.js.template
index f94bbf0..a8fed99 100644
--- a/libretranslate/templates/app.js.template
+++ b/libretranslate/templates/app.js.template
@@ -1,6 +1,6 @@
 // @license magnet:?xt=urn:btih:0b31508aeb0634b347b8270c7bee4d411b5d4109&dn=agpl-3.0.txt AGPL-3.0
 // API host/endpoint
-var BaseUrl = window.location.protocol + "//" + window.location.host + url_prefix ;
+var BaseUrl = window.location.protocol + "//" + window.location.host + "{{ url_prefix }}" ;
 var htmlRegex = /<(.*)>.*?|<(.*)\/>/;
 document.addEventListener('DOMContentLoaded', function(){
     var sidenavElems = document.querySelectorAll('.sidenav');
diff --git a/libretranslate/templates/index.html b/libretranslate/templates/index.html
index 185ce43..deead3f 100644
--- a/libretranslate/templates/index.html
+++ b/libretranslate/templates/index.html
@@ -9,15 +9,12 @@
 	<link rel="shortcut icon" href="{{ url_for('static', filename='favicon.ico') }}">
 	<meta name="description" content="{{ _h('Free and Open Source Machine Translation API. Self-hosted, offline capable and easy to setup. Run your own API server in just a few minutes.') }}">
 	<meta name="keywords" content="{{ _h('translation') }},{{ _h('api') }}">
-	<script type="text/javascript">
-		var url_prefix = "{{ url_prefix }}"
-	</script>
 
 	<link rel="preload" href="{{ url_for('static', filename='icon.svg') }}" as="image" />
 	<link rel="preload" href="{{ url_for('static', filename='js/vue@2.js') }}" as="script">
 	<link rel="preload" href="{{ url_for('static', filename='js/materialize.min.js') }}" as="script">
 	<link rel="preload" href="{{ url_for('static', filename='js/prism.min.js') }}" as="script">
-	<link rel="preload" href="{{ url_for('static', filename='js/app.js') }}?v={{ version }}" as="script">
+	<link rel="preload" href="js/app.js?v={{ version }}" as="script">
 
 	<link rel="preload" href="{{ url_for('static', filename='css/materialize.min.css') }}" as="style"/>
 	<link rel="preload" href="{{ url_for('static', filename='css/material-icons.css') }}" as="style"/>
@@ -75,7 +72,7 @@
 					<li id="change-language"><a class="noline" href="javascript:void(0)" title="{{ _h('Change language') }}"><i class="material-icons">language</i></a>
 					</li>
 					<li class="locale-panel" id="locale-panel">
-						<select id="locales">
+						<select id="locales" onchange="changeLocale(this)">
 							{% for l in available_locales %}<option value="{{ l['code'] }}" {{ 'selected' if current_locale == l['code'] else ''}}>{{ l['name'] }}</option>{% endfor %}
 						</select>
 						<a href="#TODO">{{ _h("Edit") }}<i class="material-icons">create</i></a>
@@ -83,6 +80,18 @@
 					{% endset %}
 					{{ menulinks }}
 				</ul>
+				<script>
+				var localeLinks = {
+					{% for al in alternate_locales %}"{{ al.lang }}": "{{ al.link }}"{% if not loop.last %},{% endif %}
+					{% endfor %}
+				};
+
+				function changeLocale(slt){
+					var lang = slt.value;
+					if (localeLinks[lang]) location.href = localeLinks[lang];
+					else location.href = '?lang=' + slt.value;
+				}
+				</script>
 
 				<ul id="nav-mobile" class="sidenav">
 					{{ menulinks }}
@@ -325,6 +334,6 @@
 	// @license-end
 	</script>
 	<script src="{{ url_for('static', filename='js/prism.min.js') }}"></script>
-	<script src="{{ url_for('static', filename='js/app.js') }}?v={{ version }}"></script>
+	<script src="js/app.js?v={{ version }}"></script>
 </body>
 </html>
diff --git a/requirements.txt b/requirements.txt
index 5f21574..e68dba1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,6 +4,7 @@ flask-swagger==0.2.14
 flask-swagger-ui==4.11.1
 Flask-Limiter==2.6.3
 Flask-Babel==2.0.0
+Flask-Session==0.4.0
 waitress==2.1.2
 expiringdict==1.2.2
 LTpycld2==0.42