A list. Add more.

Basque

Adhur
Aiumeen Basoa
Ilbeltz
Numen

Faroese

Tyr

Karelian

СатанаКозёл (might just be the one song, it's a Myllärit cover)

Mongolian

Ego Fall (though they mostly use Mandarin)
There is also Altan Urag, but they aren't really metal.

Sami

Korpiklaani

Savo Finnish

Verjnuarmu

Tatar

Baradj

Any Finnish band. But that's maybe because I find sole Finnish extremely interesting.

Otherwise, Leaves Eyes — Wikipedia says it's metal — tends to experiment with numerous languages, especially in the more recent albums (Meredead, Symphonies of the Night).

Greenlandic

Arctic Spirits
An incredibly shitty name even by metal standards, but their music is decent. Lateral fricatives + 80s metal vocals.

Mandarin Chinese

Ironblade