SOLVED. YA Fantasy: People have the ability to leave their bodies to observe other places. Two main characters are sisters, one has always spoken gibberish. Main character pretends the sister is one of the body-leavers to earn money. [s]

[Haleigh]

It is normal for people new to spirit walking to have trouble controlling their bodies at first, so the main character pretends to be the sisters interpreter and helps her cheat the tests which are meant to tell if a person is faking it or actually spirit-walking. The MC believes the sister is actually just disabled, or has lost her spirit outside of her body.

*Spoilers*

They are forced to go on a quest of sorts, and it is eventually revealed that the sister is not speaking gibberish, but had, in fact almost permanently left her body to spend time with a group of people who spoke a different language. The people do not recognize her (as she only visited there in spirit.) The only other thing I remember is the lava flow.

[Spockles]

This is The Lost Conspiracy (also called Gullstruck Island) by Frances Hardinge.

[Haleigh]

Yes that's it! I've been looking for it for several years now