diff --git a/config/test.exs b/config/test.exs
index fe2b855..864222f 100644
--- a/config/test.exs
+++ b/config/test.exs
@@ -49,6 +49,9 @@ config :mv, :session_identifier, :unsafe
 
 config :mv, :require_token_presence_for_authentication, false
 
+# Use English as default locale in tests so UI tests can assert on English strings.
+config :mv, :default_locale, "en"
+
 # Enable SQL Sandbox for async LiveView tests
 # This flag controls sync vs async behavior in CycleGenerator after_action hooks
 config :mv, :sql_sandbox, true
diff --git a/lib/mv_web/live_user_auth.ex b/lib/mv_web/live_user_auth.ex
index b78ba21..c913caa 100644
--- a/lib/mv_web/live_user_auth.ex
+++ b/lib/mv_web/live_user_auth.ex
@@ -42,9 +42,8 @@ defmodule MvWeb.LiveUserAuth do
   end
 
   def on_mount(:live_no_user, _params, session, socket) do
-    # Set the locale for not logged in user to set the language in the Log-In Screen
-    # otherwise the locale is not taken for the Log-In Screen
-    locale = session["locale"] || "en"
+    # Set the locale for not logged in user (default from config, "de" in dev/prod).
+    locale = session["locale"] || Application.get_env(:mv, :default_locale, "de")
     Gettext.put_locale(MvWeb.Gettext, locale)
     {:cont, assign(socket, :locale, locale)}
 
diff --git a/lib/mv_web/locale_controller.ex b/lib/mv_web/locale_controller.ex
index 99a200f..afa1737 100644
--- a/lib/mv_web/locale_controller.ex
+++ b/lib/mv_web/locale_controller.ex
@@ -1,10 +1,11 @@
 defmodule MvWeb.LocaleController do
   use MvWeb, :controller
 
-  def set_locale(conn, %{"locale" => locale}) do
+  @supported_locales ["de", "en"]
+
+  def set_locale(conn, %{"locale" => locale}) when locale in @supported_locales do
     conn
     |> put_session(:locale, locale)
-    # Store locale in a cookie that persists beyond the session
     |> put_resp_cookie("locale", locale,
       max_age: 365 * 24 * 60 * 60,
       same_site: "Lax",
@@ -14,6 +15,8 @@ defmodule MvWeb.LocaleController do
     |> redirect(to: get_referer(conn) || "/")
   end
 
+  def set_locale(conn, _params), do: redirect(conn, to: get_referer(conn) || "/")
+
   defp get_referer(conn) do
     conn.req_headers
     |> Enum.find(fn {k, _v} -> k == "referer" end)