[davismwfl]

Add a language to your CV when you are fluent enough in the language to understand all the syntax and you can read anything written in the language and you have written code yourself in it. This doesn't mean you are an all out expert and know every detail, just that you are capable in the language. Most of the time we use 60-80% of a programming language IME, so once you know that common set you are good. That other 20-40 percent of a language is usually specialized to specific things or fringe and not common, so not knowing those fluently is fine IMO.

Put another way, you know a language when you can pick up any project written in it and can read it intelligently without having to look up what common library functions do (not all of course, but the most common of the functions).