Make language selection sticky and provide a way to return back to default auto-detect #3792

changedetectionio/blueprint/ui/__init__.py

@@ -404,4 +404,25 @@ def construct_blueprint(datastore: ChangeDetectionStore, update_q, worker_handle
 
         return redirect(url_for('watchlist.index'))
 
+    @ui_blueprint.route("/language/auto-detect", methods=['GET'])
+    def delete_locale_language_session_var_if_it_exists():
+        """Clear the session locale preference to auto-detect from browser Accept-Language header"""
+        if 'locale' in session:
+            session.pop('locale', None)
+            # Refresh Flask-Babel to clear cached locale
+            from flask_babel import refresh
+            refresh()
+            flash(gettext("Language set to auto-detect from browser"))
+
+        # Check if there's a redirect parameter to return to the same page
+        redirect_url = request.args.get('redirect')
+
+        # If redirect is provided and safe, use it
+        from changedetectionio.is_safe_url import is_safe_url
+        if redirect_url and is_safe_url(redirect_url):
+            return redirect(redirect_url)
+
+        # Otherwise redirect to watchlist
+        return redirect(url_for('watchlist.index'))
+
     return ui_blueprint

changedetectionio/flask_app.py

@@ -94,6 +94,14 @@ if os.getenv('FLASK_SERVER_NAME'):
 app.config['BABEL_TRANSLATION_DIRECTORIES'] = str(Path(__file__).parent / 'translations')
 app.config['BABEL_DEFAULT_LOCALE'] = 'en_GB'
 
+# Session configuration
+# NOTE: Flask session (for locale, etc.) is separate from Flask-Login's remember-me cookie
+# - Flask session stores data like session['locale'] in a signed cookie
+# - Flask-Login's remember=True creates a separate authentication cookie
+# - Setting PERMANENT_SESSION_LIFETIME controls how long the Flask session cookie lasts
+from datetime import timedelta
+app.config['PERMANENT_SESSION_LIFETIME'] = timedelta(days=3650)  # ~10 years (effectively unlimited)
+
 #app.config["EXPLAIN_TEMPLATE_LOADING"] = True
 
 
@@ -550,6 +558,9 @@ def changedetection_app(config=None, datastore_o=None):
 
         # Validate the locale against available languages
         if locale in language_codes:
+            # Make session permanent so language preference persists across browser sessions
+            # NOTE: This is the Flask session cookie (separate from Flask-Login's remember-me auth cookie)
+            session.permanent = True
             session['locale'] = locale
 
             # CRITICAL: Flask-Babel caches the locale in the request context (ctx.babel_locale)

changedetectionio/templates/base.html

@@ -265,6 +265,11 @@
           </a>
           {% endfor %}
         </div>
+        <div>
+            <a href="{{ url_for('ui.delete_locale_language_session_var_if_it_exists', redirect=request.path) }}" class="language-option" style="margin-top: 1em; display: block;">
+              <span class="lang-option">🌐</span> <span class="language-name">{{ _('Auto-detect from browser') }}</span>
+            </a>
+        </div>
         <div>
             {{ _('Language support is in beta, please help us improve by opening a PR on GitHub with any updates.') }}
         </div>

changedetectionio/tests/test_i18n.py

@@ -160,7 +160,7 @@ def test_invalid_locale(client, live_server, measure_memory_usage, datastore_pat
 def test_language_persistence_in_session(client, live_server, measure_memory_usage, datastore_path):
     """
     Test that the language preference persists across multiple requests
-    within the same session.
+    within the same session, and that auto-detect properly clears the preference.
     """
 
     # Establish session cookie
@@ -184,6 +184,32 @@ def test_language_persistence_in_session(client, live_server, measure_memory_usa
         assert res.status_code == 200
         assert b"Annulla" in res.data, "Italian text should persist across requests"
 
+    # Verify locale is in session
+    with client.session_transaction() as sess:
+        assert sess.get('locale') == 'it', "Locale should be set in session"
+
+    # Call auto-detect to clear the locale
+    res = client.get(
+        url_for("ui.delete_locale_language_session_var_if_it_exists"),
+        follow_redirects=True
+    )
+
+    assert res.status_code == 200
+
+    # Verify locale was removed from session
+    with client.session_transaction() as sess:
+        assert 'locale' not in sess, "Locale should be removed from session after auto-detect"
+
+    # Now requests should use browser default (English in test environment)
+    res = client.get(
+        url_for("watchlist.index"),
+        follow_redirects=True
+    )
+
+    assert res.status_code == 200
+    assert b"Cancel" in res.data, "Should show English after auto-detect clears Italian"
+    assert b"Annulla" not in res.data, "Should not show Italian after auto-detect"
+
 
 def test_set_language_with_redirect(client, live_server, measure_memory_usage, datastore_path):
     """