Related gripe: Twitter will only offer the report form (for e.g. a harmful tweet) inside Germany, in German. I do understand conversational German but not German legalese; I will not bother to select which exact subparagraph of the communications legislation the tweet violates, I will just close the tab and let someone else report it.
Union Jack? English Flag? USA Flag? Some horrible hybrid between them?
None of them is a perfect fit. As a techie I'd say ISO codes ('en', 'en-gb', 'fr', 'de') but I'm not sure how much those are understood, and probably not so hot for non-latin scripts.
This lets you specify an ordered list of whatever languages you prefer. If you haven't set it manually, usually it defaults to whatever your system language is.