Languages
In general--
Pick one or the other. One does not just "pick up" Arabic or Chinese. Arabic, though, is generally easier for an English-speaker to learn.
Benefits in the military--
If you learn either language to a level of proficiency, you can take a listening/reading comprehension test once you're in the military and get extra monthly pay. Both of these languages pay very well (at least $500 a month I think, but don't quote me on that) for the highest levels of proficiency, and they are both in the top tier of language pay. And it's a tough test. I only got the second-best scores on languages in which I considered myself very fluent. Other than that, you probably won't be called upon to use your language skills through your first aviation tour. After that, it depends on your choices and the needs of the Navy.
Civilian--
Again, mostly it will depend on your choices and the current business environment. Back in the early 1900s, one of the most in-demand languages in the diplomatic and business world was Turkish. One little skirmish in Europe changed all that. Also, think about this...What you learn might determine where you live. And what if you select Prowlers and have a bunch of little girls?

Would you want to raise them in Arabic-speaking countries?