i18n: add DEFAULT_UI_LANGUAGE server setting

The DEFAULT_UI_LANGUAGE server setting allows to override the "Use browser's default" when detecting the locale to use.
This allows to e.g., force the default language of the users server-wide, which can be useful in specific use cases (e.g., exams).

timApp/defaultconfig.py

@@ -179,6 +179,12 @@
 SESSION_COOKIE_SAMESITE = "Lax" if HAS_HTTPS else None
 SESSION_COOKIE_SECURE = HAS_HTTPS  # Require HTTPS or localhost for session cookies
 
+DEFAULT_UI_LANGUAGE = None
+"""
+The default UI language to use when user's language is not explicitly set. 
+If not set, determines the language based on the user's browser settings.
+"""
+
 BOOKMARKS_ENABLED = True
 
 # If False, only admins can create folders and documents.

timApp/util/locale.py

@@ -12,13 +12,18 @@ def get_locale(force_refresh: bool = False) -> str:
         KNOWN_LANGUAGES, default="en-US"
     )  # type: ignore
     lng = request.cookies.get("lang")
+    default_language: str | None = current_app.config["DEFAULT_UI_LANGUAGE"]
+    if default_language not in KNOWN_LANGUAGES:
+        default_language = None
     if not lng or force_refresh:
         if not logged_in():
-            return header_lang
+            return default_language or header_lang
         u = get_current_user_object()
         lng = u.get_prefs().language
     if lng in KNOWN_LANGUAGES:
         header_lang = lng
+    elif default_language is not None:
+        header_lang = default_language
     override = get_document_lang_override()
     if override and override in KNOWN_LANGUAGES:
         header_lang = override