If you cannot find a full pack, you can extract the English voices from a legitimate Steam installation (from a friend or secondary PC). Use (unpacker for Far Cry 3). Extract the English .dat files, then repack them into a common.dat that matches your regional executable. This requires technical proficiency and is not recommended for casual users.
: If the game is stuck in another language (common in Russian versions), you can force it via the Windows Registry. English Language Pack For Far Cry 3