Can't reset language - Internal Error

I could reset all my languages except for German. Every time there is a pop up window that says: “Internal Error”. I’ve deleted cookies and tried other browser (Chrome and Firefox). What’s wrong?

@session13 - This seems to happen in some cases and is based on the type of information that is stored in certain language slots. In short, it won’t be possible to delete or reset these languages. However, there isn’t really a reason to need to do this, as you can just highlight any words you marked as Known to save them again or delete any lessons you’ve taken to return them to the Library.