Changing language files manually is generally safe, but ensure the files are from a trusted source to avoid issues with anti-cheat software on some servers.
C:\Program Files (x86)\Steam\steamapps\common\Call of Duty 2\main Remove Regional Files : Files named call of duty 2 english language pack exclusive
The Logistics of Digital Exclusion: An Analysis of Region-Locked Language Packs in Call of Duty 2 Changing language files manually is generally safe, but