And now that everyone's done being sarcastic, no. It's simple enough to automatically label someone's native tongue based on region and giving them a checklist of languages they might also speak if they want to dig in the options, there's no need to make that question more complicated over a subjective matter. Either you say you can speak a language or you can't, either you have enough confidence to play with native speakers or you don't. Profeciency tags would ultimately mean nothing.
To be honest, the best solution I can see is just having quick voice (or chat) commands. Having a bunch of flags next to everyone's names means you're identifying everyone by their nation first and foremost, there's a huge risk that that could spark some very nasty nationalism. It's just that I can very much see the chat commands not covering enough or being too clunky for new and confused players to use.
