Firefox's new offline translation is certianly a nice feature.
My question is, does clicking on these buttons install non-free software on my computer?
I don't think that the free software community has settled on an answer to this question.
FWIW, I do own copyright to data used in this translation. They have ingested the translations of the debian installer into it; I wrote a lot of that text and it is GPL licensed.
I found this url in their training set, for example: