You should learn languages that meet two conditions (1) that are in danger of dying out, and (2) represent rare types of languages. Learn Basque, because it's dying out and had no close relatives. Don't learn Ladino or Gagauz because although they are dying out, numerous closely related Indo-European or and Turkic languages remain. Learn any Native American languages, or Khoisan languages (not Xhosa, which is a Bantu language, and Bantu languages are nowhere near to dying out), or Australian Aboriginal languages. If you want to learn an Indo-European language, learn a Celtic language because all those are dying out.