There's a book I'm trying to remember. It follows a boy from a tribe in a world of monsters, their brother is . The tribe comes across a woman that speaks a different language and they take her in as a slave. As they get to know each other he treats her kindly and at a point they find a universal translator. The woman is actually from a group of people that live in a dome around the surface of the Earth while the tribe lives under an artificial sky.
I think this is The Inferior by Peader O'Guilan, the first of his Bone World trilogy
Blurb from Fantastic Fiction:
STOPMOUTH AND HIS family know of no other life than the daily battle to survive. To live, they must hunt rival species, or negotiate flesh-trade with those who crave meat of the freshest human kind. It is a savage, desperate existence. And for Stopmouth, considered slowwitted hunt-fodder by his tribe, the future looks especially bleak. But then, on the day he is callously betrayed by his brother, a strange and beautiful woman falls from the sky. It is a moment that will change his destiny, and that of all humanity, forever.