For PC players, Steam is the safest and fastest way to acquire the English language pack.
Before trying anything technical, try the built-in Steam setting: Right-click Black Ops 3 in your Steam Library. Properties from the dropdown menu. Black Ops 3 English Language Pack Download
If you bought a key that is strictly region-locked (e.g., a "CIS" version meant for Russia/CIS countries), the English option may be permanently removed from the Steam properties menu. In this case, players often look for a manual . For PC players, Steam is the safest and