We were having a conversation about linguistic sci-fi the other night in the pub (best place for it, I find). I love sci-fi, and I love linguistics, and I once read a list by Geoff Pullum (in the collection of his columns, 'The great Eskimo vocabulary hoax') of books that combine the two.
To count as linguistic sci-fi, language/linguistics has to be relevant to the story, in my opinion. It's not enough that they just speak a different language, like Klingon in Star Trek. Although there is some linguistic aspect, as Klingon is obviously supposed to sound aggressive, you couldn't say that Star Trek is linguistic sci-fi just because of that. To count, the language has to be central or at least important to the plot.
The books on Pullum's list turned out to be surprisingly hard to get, but this is one I did find on Amazon:
It wasn't that great, to be honest. The basic idea is that someone wants to control the people of Pao, who are docile and peaceful (well, not warlike anyway). To control them and make them a bit more fighty, the scientists at another planet, Breakness, have devised a new language which will allow them (force them?) to be less placid. It's obviously all very Sapir-Whorfian, and an interesting idea. It's unfortunately not really my kind of book, being the kind where the characters must flee things and avenge people and wear big weapons and so on.