As ATranswarp pointed out in another thread, Hoshi has to be savvy on the technical side of her job too, ie. the Universal Translator and all the comm systems on board ship. That HAS to be a given, if she's the head of the Communications dept. Just because we didn't see her actually *doing* that on the series doesn't mean she *can't* do it.
Codebreaking...that's definitely another skill set. Just because you're trained in linguistics doesn't necessarily mean you're automatically a cryptologist. But it can help. There are patterns common to almost all languages (and Hoshi admits to Trip in "Observer Effect" that she's good at picking them up and "Math is just another language"). In order to construct a translation matrix (like the UT) you have to spot those patterns and program them into a form that will help decode/translate a foreign language into something intelligible. So translating a language is like breaking a (foreign) code in a certain way. If Hoshi was supposed to encode the Universal Translator matrix (according to her "future profile" in IAMD), she'd better be well-versed in a)picking up the patterns and b)technically competent enough to program the matrix in the first place.
That's another branch of linguistics, called COMPUTATIONAL linguistics, and that's exactly when it sounds like. Analyzing all the possible permutations of language. It wouldn't surprise me if Hoshi was also skilled at (or at least, familiar with) this. The more cultures Enterprise meets (and the more First Contacts they make), she'd have to use these skills to at least get the basic communications down. And no, it probably wouldn't happen as fast as it does on the show, but that's Star Trek.
So I think Hoshi really does represent a "communications officer" on the first warp 5 starship. One thing I really didn't like about Uhura in TOS was that they showed her as nothing more than just s glorified switchboard operator.

(Annie, who specialized in foreign languages, applied linguistics and education)