NC for this. We live in DH's home country. I am English and buy the DC books in English. I would say 90% of their books are ones I have bought. No access to a library with English books. We live in an area near a border (bear with me, this gets complicated), where IL's native language is spoken. This is therefore the language of DC's school. IL's live across the border where DH's native language is spoken, DH also works there. Each speak their own language to the DC and DH speaks IL's native language to his parents and his own native language to BIL (except when the parents are present).
DH and MIL have several times commented that the DC don't have many books in their respective languages. I have bought a few but am a bit lost at deciding which ones are worth buying. I have ALWAYS made sure that half of the kid's bookshelf in the living room has books in MIL's language so she has something she can read to them. Turns out it's part MIL not wanting to go through the bookcase 
and the DC choosing their favourite books to look at with her. DC also bring home books from the school library.
I always suggest she buys them some books when she asks what they want as presents. DH eats lunch in a supermarket complex so has plenty of opportunity, (and the internet,) to find books for them. He works with people with slightly older children so could ask around for recommendations.
AIBU to tell him he should stop complaining
a) that the DC understand/speak his language worst of all
b) that we have comparatively very few books in his language
c) and that he should find some books himself to read to them?
(At the moment he usually reads English books to them at bed time.)
Finally, that he should gently suggest to his mum that she is perfectly welcome to find some good books for the DC?