From what I head prior to enlisting... Taking the DLAB is required, but there is no minimum score for 18X entry...
I got a 121 or 124 (not sure) and I thought it was crap because my buddy got 138(bastard) and I didn't have anyone else to compare myself with. To me, it seemed more like an analytical IQ test than anything else. There's probably a good chance you could learn languages faster than I could. If you're already bilingual, then follow the route psywar suggested... If not, don't worry about it. Just give it 100% and see what happens. What we lack in aptitude we can make up for in effort.
Good luck!!