Change it to: seta locale "english"
If English is not listed, you likely own a "Region-Locked" copy (often labeled ROW or RU/CIS). If this is the case, proceed to Method 2.
For regional versions (such as Russian or Polish) that may not show English in the Steam settings, players often use manual file swaps.
(and several other older CoD titles) often bundled specific localizations into the core files of regional retail copies to prevent "gray market" key reselling. Quick Troubleshooting Incompatible Data Error
Some versions require editing the configuration file. Look for config.cfg or configs.user and ensure the line seta sys_lang "english" or language=english is present.