From Broken Bow:
: What do you know about these Klingons?
: Not much. An empire of warriors with eighty poly-guttural dialects constructed on an adaptive syntax.
The Vulcan spoken by T'Pol, spoken by Spock (and the Amok Time wedding party) and spoken by Tuvok might have all been different languages from each other, with the exception of a very few common words and terms, like Pon Farr.
The Vulcan Priestess at the beginning of TMP might have been speaking a forth language, perhaps a ancient language like Latin.