If you meant the 2023 Modern Warfare III (by Sledgehammer Games), language packs are managed exclusively via Call of Duty HQ (Battle.net or Steam) → Settings → Modify Install → Language – no manual file swapping is needed or supported.
The English Language Pack for Call of Duty: Modern Warfare 3 includes: Call Of Duty Modern Warfare 3 English Language Pack
As of 2025, official support for Modern Warfare 3 is minimal. However: If you meant the 2023 Modern Warfare III
PC players have the most control over specific file installation through their respective launchers. On Battle.net: Select in your library. Click the Gear Icon (Settings) next to the Play button. Select Modify Install . Call Of Duty Modern Warfare 3 English Language Pack
Choose the English language pack from the available "Manage Installation" list.