I know a linguist that works on Flathead (Sally Thomason)- language is dying out as elders do. Lakota, OTOH, has many more people speaking, so it won't die out tomorrow- it is also taught in schools. I had a Lakota informant (a term these days not PC) back in graduate school. If you ever want to learn materials are relatively inexpensive. Grammatically Siouan languages are somewhat simple, but they make up for that with sometimes extreme lexical elaboration. Flathead the other way around.

Jess Tauber