[Meai]

interesting though, that comment directly says that if I tweet in english and my interface is in german then I'll have a serious problem. Which I wouldn't think is so rare. How many people have english as their second language and use it online? It feels like this deboost would hit a lot of people. Or actually, I have my interface in english and I often tweet in german. So that would hit me from both sides.

[derefr]

"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.

[JonathonW]

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).