There are a great deal of math articles/books in Latin, French, and Russian that have no English translations at this point.
For example, Euler wrote 800 articles (in Latin and French) and only 207 have been translated into English.
- http://eulerarchive.maa.org/enestrom.php?topic=trans (link 404s once in a while, so worth refreshing a bit later if it doesn't work)
Examples of works in the French language:
Examples of scans of Math articles that may or may not be in English:
- https://dmitripavlov.org/scans/
A French example from that archive of scanned math articles