While modifying localization files is generally considered safe and players have reported no bans for language packs, always use caution when modifying game directories while Steam is active.
What are you playing on? (Steam, console, or an alternative client like Boiii?) black ops 3 language switcher download
If you have the files but the game refuses to launch in the right language, you can manually "switch" the setting in the game's configuration. right-click the game in Steam
If Steam automatically downloads the old language files again, right-click the game in Steam, go to Properties > Updates, and turn off automatic updates, or set the modified configuration files to "Read-Only" by right-clicking the file, selecting Properties, and checking the Read-Only box. Conclusion go to Properties > Updates