If the in-game and launcher options aren't working, you can try manually editing the configuration files. This is a common solution mentioned by players in forums.
You will need to download the missing .dat and .fat language files from a verified community source or backup your files from a worldwide edition. Look for files named similarly to: sound_english.dat and sound_english.fat lang_english.dat and lang_english.fat far cry 4 language pack
Many searches for "Far Cry 4 language pack" come from gamers who already speak English. Why change? If the in-game and launcher options aren't working,
The Ultimate Guide to Far Cry 4 Language Packs: How to Change and Install Voice and Text Files far cry 4 language pack