I am a sixth grade teacher looking to help a student. My kids are doing a project on a civilization of their choosing, and despite a lot of effort we cannot find any info on the written language of Mali. I'm assuming Arabic, I know it was a linguistically diverse empire but I don't know if the other languages were written at that point.
I think most of the writing that's survived in the sahel is arabic, the mauritanian manuscripts are and the most famous timbuktu manuscripts are too.
However, according to University of Cape Town the Timbuktu manuscripts do also contain works in Songhay, Fulfulde, Tamasheq and Hausa.
The problem is the manuscripts aren't all from the mali empire time, some are from later and some earlier and as far as I am aware they haven't been catalogued enough for anyone to be able to say for sure when those languages were first used and if that was after or before the mali empire lost timbuktu (Timbuktu was recently captured by islamic rebels and the cape town university group working at it lost funding so a lot of work is still to be done in studying the manuscripts).
Possibly someone who'd actually been to timbuktu would be able to tell you for sure.