Japan has a second, wholly unrelated indigenous language that's dying out

Ainu, spoken by the indigenous people of Hokkaido and nearby islands, has no proven relationship to Japanese or any other living language — it's a genuine isolate. Decades of forced assimilation policy left it critically endangered, with only a handful of fully fluent elderly speakers believed to remain.

— Wikipedia contributors, Ainu language — UNESCO Atlas of the World's Languages in Danger