Theoretically speaking, I'd put Hawkes as more likely to know another language than Stella - but as it turns out, she's the one who speaks other languages, and we have no information to indicate that Sheldon does (although it would be interesting if he perhaps knew Spanish, which would be helpful if he had a lot of Spanish-speaking patients during his time in the ER, or something like that).
While a good percentage of New Yorkers are bilingual (a larger percentage than in some other parts of the country, certainly), just because Stella, Angell, Flack and even Sid spoke another language doesn't mean every single member of the cast should get in on the bilingual action. (If they did, we'd be in here bitching about more overkill in the Diversity department.)
If they'd made the characters bilingual based on what is most logical for the various characters, I'd say Hawkes and Danny would be the most likely candidates to be bilingual (although I could be convinced otherwise) - but instead, Stella's knowledge of Greek, like Angell's knowledge of French, was dictated by the actors. That's not always a bad thing, I'm just sayin'. (It's certainly not as questionable as the myriad things about Danny that are influenced by Carmine rather than what is most logical for the character.)
In any case, Hawkes wouldn't be any less of a genius if he didn't know another language.
Until he busts out some Mandarin in the middle of a conversation, there is no more proof that he's bilingual than there is proof that he only speaks English. (Logic doesn't count - this is CSI:NY, logic never counts.)
