The short of it: when you feel comfortable answering questions about the language (which implies you've spent some time with it and know a fair deal about it). I know python and ruby, but not everything there is to know about it; I could probably be stumped by the implementation of python's enumerate() function in C (which I also "know"), but I know what enumerate does and what it's good for.
If you're looking to solidify your resume, think about this. You mostly want to have on your resume the things that you want to work with. Then people will ask you things about the language in interviews and if you don't know the answer, that's fine. At that point you can just find out what the answer is (possibly from the interviewer) and now you have it for the next time you get asked. That's what I did last time I was job hunting. I learned a lot from my interviews.
I also wouldn't be afraid to put something like "some C" on a resume, if you're looking to improve upon it. It'll make for a conversation in interviews.
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