If you own the Steam version but your store region limited the language:
File naming uses language codes: en, en_us, en_gb, fr, de, es, etc.
Back up any files before modifying. Editing or replacing game files can break the game or violate TOS.
Steam hates modified files. It may see the new English pack as "corrupt" and immediately download the old Russian files again. Far Cry 4 English Language Pack