Treyarch shipped Black Ops 2 with strict language-locked executables. If you bought the game in a French territory, the Steam CDN (Content Delivery Network) will only deliver French audio and subtitle files. Changing the Steam language setting might revert your entire Steam client language, but the game’s core files often remain unchanged or cause the game to crash due to mismatched hash checks.
If you don't have a switcher folder, you must manually download and replace the localization files.