For as long as there is language, there will be "language learning". We are not just born with full fluency in a language and AI, as OP has described it, will not change that. Furthermore, for as long as people of different cultures make non-language-knowing babies with each other, at least some of them will be multi-lingual.

Final prediction: a few thousand years at best (if humanity makes it) (but in the event we don't make it, that may be the only scenario in which language learning is no longer necessary. Because there won't be language at all)