If you have Uplay installed on your computer, you can also download the language pack through the client:

Whether you need to tweak a single line of code or download the missing voice assets, getting Far Cry 3 running in English is a quick fix that ensures you can fully enjoy Vaas Montenegro’s iconic performance as intended.

If all else fails, remember the Ubisoft Connect method is your safest bet, followed by the vetted ModDB community uploads. Do not settle for a dubbing that breaks the immersion. Vaas’s insanity speech, Jason’s transformation, and Buck’s chilling menace are meant to be heard in English.

from a reputable fan repository or modding site (e.g., Nexus Mods, ModDB, or a community thread on Steam). Ensure the pack matches your game version (typically the standard DX11 release).