"One of the user's understandable language" presumably refers to the list of languages in the browser's Accept-Language header. If you are tweeting in English, you'd likely have English in that header.
It may or may not use the Accept-Language header as well, but this is a user setting accessible via the Twitter web UI-- you specify your primary interface language and can specify any number of additional languages that you understand.
There's also a place in Twitter's settings to show languages that Twitter has inferred that you know (e.g. following a German-language account and interacting with it almost certainly means that you know some German).
