I think this is an excellent project, so excellent that I have to ask, why are we not (if indeed we are not) already doing this, all the time?
Weight-watcher groups are watching their weight all the time, not just meeting to talk about it. People meeting to help each other learn a foreign language are learning that language the rest of the time, at least, for as many hours a day as they find useful. University students make studying a full-time job (the ones that are serious). Rationality is supposed to be applicable to everything; every moment is an opportunity for practice.
You mentioned specific groups which try to reach a specific goal. That's great, on the level of individual goals. But we also need to go more meta. The foreign language group will not tell you to stop learning the language if your life situation changes so that the original purpose of learning the language is no longer valid; or if simply a better opportunity appears and it would be rational to move your limited resources from the language towards something else. Also, if you already haven't decided to study a specific language, the group will not find you...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.