"Open Educational Resources" by Ron Mader is licensed under CC BY 2.0
OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. ~ William & Flora Hewlett Foundation
For a more comprehensive overview on OER, see:
"Language of the World" by M. Adiputra is licensed under CC BY-SA 4.0
This list contains samples of several OER World Languages resource collections. Specific materials (such as textbooks) can be found on the language's Web Resources tab.
Open Access materials can be found using the Mason OER Metafinder. Try using the language as keyword (ex. Japanese) and filter from there.