If you’ve ever complained about the difficulty of learning to pronounce a foreign language, spare a thought for Ubykh.
Among other peculiarities, its outstanding feature has to be the fact that it has—or rather had—eighty-three consonants, yet only two vowels. It also possessed a fearsome grammar:
Its system of verbal agreement is frighteningly complex. English verbs must agree only with the subject; Ubykh verbs, by contrast, must agree with the subject, the direct object and the indirect object, and benefactive objects must also be marked in the verb.
I can’t imagine why it died out a couple of decades ago.