diff --git a/app/src/web/i18n.py b/app/src/web/i18n.py
index ddbee9b..918e6b8 100644
--- a/app/src/web/i18n.py
+++ b/app/src/web/i18n.py
@@ -1,4 +1,4 @@
-from quart import request, session, redirect
+from quart import request, session, redirect, abort
 from quart_babel import _
 from quart_babel.locale import get_locale as babel_get_locale
 
@@ -39,4 +39,5 @@ def utility_processor():
 def set_language(language=None):
     if language in LANGUAGES:
         session['language'] = language
-    return redirect(request.referrer)
+        return redirect(request.referrer)
+    return abort(400)
diff --git a/app/tests/test_web_route.py b/app/tests/test_web_route.py
index ff5e8c2..d0a3d35 100644
--- a/app/tests/test_web_route.py
+++ b/app/tests/test_web_route.py
@@ -124,3 +124,34 @@ async def test_data_fetch2(create_inverter_client):
 
     response = await client.get('/data-fetch')
     assert response.status_code == 200
+
+@pytest.fixture
+def client():
+    app.secret_key = 'super secret key'
+    return app.test_client()
+
+@pytest.mark.asyncio
+async def test_language_en(client):
+    """Test the language/en route."""
+    response = await client.get('/language/en')
+    assert response.status_code == 302
+    assert response.mimetype == 'text/html'
+
+    client.set_cookie('test', key='language', value='de')
+    response = await client.get('/page')
+    assert response.status_code == 200
+    assert response.mimetype == 'text/html'
+
+@pytest.mark.asyncio
+async def test_language_de(client):
+    """Test the language/en route."""
+    response = await client.get('/language/de')
+    assert response.status_code == 302
+    assert response.mimetype == 'text/html'
+
+@pytest.mark.asyncio
+async def test_language_unknown(client):
+    """Test the language/en route."""
+    response = await client.get('/language/unknonw')
+    assert response.status_code == 400
+    assert response.mimetype == 'text/html'