(Why is this a basic scuba disussion?)
I think the main thing to concern yourself with in terms of language skills is will you (be able to) learn enough to directly brief divers so that you are not having to use the one English speaker to relay your brief to the team?
And then on the extreme end, can you handle an emergency in the language, especially in the worst case of the English speaker being the victim?
English skills fall away fast when bad things happen anyway. Watching Japanese unable to read Romaji (roughly Japanese written in English letters) written on a slate underwater when they can read any old Kanji written in the air in the same circumstances is a perfect example.
We Americans often forget that the cultural divide is greater than the langauge divide for us from most of the world. Linguists poo-poo the Sapir-Whorf Hypothesis in general, but the lines of personal space and conceptualizing are reflected in languages. Some of the casual topics in Japanese conversation would cause fistfights in English, and levels of address, and the hierarchical system are difficult to comprehend let alone effectively use without being fluent in Japanese. (Japanese is what I know.)
Also it is worth pointing out that although PADI has many language for materials and releases and whatnot, the time lag for getting new stuff out there translated means that unless you are going to learn to read and write the language, learning speaking it will leave in many situations where you cannot ascertain whether liability forms and health checks are actually filled out to the degree to which they would protect you. The Russian forms are a nightmare for randomly shuffling the order of the blanks.
If you have to do this professionally, then get the tear-free paper from Xerox, (not the regualr Rite-On rain which is not waterproof.
And print out briefs, underwater communications, and emergency info in the native script so you can always use that. And keep a couple of the medical forms and liability forms printed on the same paper around so you can figure out which blank needs what.