For 90% of players, the "Black Ops 3 Language Switcher" is already installed on your computer—it is the Steam Properties menu.
Many players prefer the Japanese voice acting for the "Shadows of Evil" or "The Giant" maps. However, using the official Steam method (Method 1) changes the language for the entire game.
only to find the menus and dialogue stuck in a language you don't speak, you aren't alone. This is a common hurdle for players who purchased regional versions (like the Russian or Japanese editions) that lack a standard in-game toggle.
Before downloading large file packs, try these quick text edits in your game directory: Localization File localization.txt