A vanished language leaves questions that no surviving dictionary can completely answer.

When a language stops being spoken, the words can be recorded and the use cannot. Nobody can tell you which term was affectionate, which was slightly rude, what a phrase meant among friends after midnight, or which joke turned on a double meaning. The dictionary preserves the vocabulary and loses the living register, so an entire way of meaning things goes with the last speaker.

If there is a language in your family thinning out, record someone using it rather than listing words. Conversation, argument, teasing. That is the part no reference book can reconstruct afterwards.