[jkjnwtcv] Not inventing an alien language

Instead of inventing an alien language for some science fiction or fantasy setting, use an already existing foreign language.  The advantage is that the "alien" language will have the depth and "beauty" of something that has been developed by a population over centuries, instead of a language invented on paper by one or a few writers.

The disadvantage, for those who understand the foreign language, is cognitive dissonance of, "Why do the Klingons speak (say) Estonian?"  However, perhaps this disbelief can be suspended much like every other literary conceit: "Why do the Klingons look exactly like humans in different costumes?"

Perhaps to make it a little alien to those who understand the language, use a different register of the language than one would normally expect.  Perhaps more formal or poetic, giving a job to perhaps one of the few writers who can write that way in an obscure foreign language.  (Vaguely inspired by the non-standard register (even for its time) chosen by the authors of the King James Version of the Bible.)

Of course, if the show (etc) ever gets translated to that language, then have the aliens speak English.