🥅 Don't send None to Locale.parse

The latest version of Babel no longer supports this
but raises an error, which in turn leads to a
500 Server Error response here.

To fix this, we set a default for best_match.

Closes #4744

src/octoprint/server/__init__.py

@@ -1349,7 +1349,7 @@ def _get_locale(self):
         ):
             return Locale.negotiate([default_language], LANGUAGES)
 
-        return Locale.parse(request.accept_languages.best_match(LANGUAGES))
+        return Locale.parse(request.accept_languages.best_match(LANGUAGES, default="en"))
 
     def _setup_heartbeat_logging(self):
         logger = logging.getLogger(__name__ + ".heartbeat")