Testing with Opera GX (at least), I found here that it's Opera's translate feature that automatically adds languages to Opera. With that said, there could be a bug where the newly-added language gets moved to the top of opera://settings/languages or "display Opera in this language" gets set for it.
Or, since adding a language in Opera changes Opera's Accept-Language HTTP request header to Accept-Language : en-US,en;q=0.9,fr;q=0.8 for example (in the case of French being added), some site might be incorrectly handling the priority of the languages in the header.