Note: Modifying game files may violate the EULA. Use at your own risk.
Even with a perfect language pack, issues arise. Here are the top three fixes:
Problem 1: "Sound is missing entirely"
Problem 2: Zombies Mode has Russian subtitles but English voices
Problem 3: The game crashes on launch (Error: "Unhandled exception caught")
There is a popular third-party client called Plutonium. While primarily used for anti-cheat and dedicated servers, it also includes an option to download English assets regardless of your original regional key. This is arguably the safest modern method, though it bypasses official Steam servers.
The language files for BOII are stored in the game’s root directory and within .iwd (InfoWAD) archives. The pack typically consists of: