Modifying game files is generally safe, but you should back up your original data_win32 folder before proceeding. We do not condone piracy. The language pack typically consists of files you legally own if you own the base game—you are merely "unlocking" the English audio included in the global build of the game.
Sometimes the game has the files but is "forced" to another language. You can open , navigate to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Ubisoft\Far Cry 4 , and change the 4. Direct Support Far Cry 4 English Language Pack