There are indeed very few people who speak Latin in Star Trek. But Jean-Luc Picard happens to be one of them.

When he got assimilated in 2366 and his mind became a part of the collective consciousness, the Borg gained his knowledge of this ancient language (oh, and some tactical stuff as well).

Locutus of Borg became a voice for the Borg towards the Federation, as the Latin name implies.