Most are, but it's hard to get civilians to deploy to places where they might get shot at. And at least some ammount of translation must be done in those areas. Captured documents, intercepted communications, that sort of thing.
Not all civilians of course, but the sort that might have Arabic language skills are likely to be of the sort that wouldn't want to deploy.
Security clearance. Linguists are Military Intelligence. They have access to secret squirrel stuff. Civilians can get it (clearance) but military linguists should be deployable to combat zones, at least in some numbers, or else what's the point?
There's different levels of linguists as well, from my understanding, depending how well they did in the DLI. Simultaneous translation is apparantly the hardest level. Other people do well dealing with written material but not as well with the spoken language. Some wind up being interrogators- others wind up sitting in front of radios and listening to enemy transmissions.
Don't take any of what I say as "the word" though. I had the option to become a linguist (I did well on the test) when I joined but wanted to be infantry instead. I've garnered what little I know from talking to people who went to the DLI.
No. These guys, for the most part, are not just guys sitting in cubicles translating docs...
They are involved in investigations, interrogations, and working as theatre liason situations, in other words, it isn't something civvies can do