Over in https://bugs.debian.org/1028570, i observed that the licensing for these files is not compatible with the debian free software guidelines (DFSG).

Is it possible to relicense it from CC 4.0 BY-NC-ND to just CC 4.0 BY? or CC 4.0 BY-SA?

I contributed 015-arabic/* and I'm certainly fine with those files being relicensed in such a way. I'm not sure of the origins of the other files in question.

Without relicensing, it's going to be a challenge to ensure that the package's tests run correctly in Debian, since all build and test infrastructure in the project is intended to use only DFSG-free material.