If you purchased a digital or physical copy from Eastern Europe (RU/PL) or certain Asian markets, Activision often hard-codes these versions to prevent gray-market key reselling.
The game uses a specific text file to determine which language assets to load. Replacing or deleting this can sometimes trigger a reset: localization.txt in your main BO3 folder. Open it and ensure the first line simply reads If that fails, some users find success by localization.txt cod black ops 3 english language pack work
Before trying manual file edits, check if Steam can handle the swap automatically. If you purchased a digital or physical copy