The simple truth behind out of scope detection is that it is, certainly to my understanding, an unsolved problem. I’ve written down some technical details on why in this forum post but you might also appreciate this algorithm whiteboard video on fallback detection for some extra details.
One thing you might consider doing is to have multiple types of out_of_scope. If you have a look at our rasa-demo you’ll notice that you can pre-define many types of out-of-scope that should be detected. In your case, you might be able to have out-of-scope classes for each language.
This is a path that’s reasonable, but I wouldn’t spend too much time on it immediately. The fact that there are many out of scope situations imaginable doesn’t mean that they actually occur. It’s still best to look at examples from actual users as a source of inspiration for out-of-scope categories.
Out of curiosity, when you send the response to the user, how do you determine the correct language/text to send back? Is this handled by a custom action?