1) Go to:
C:\Program Files (x86)\Steam\SteamApps\common\Carmageddon TDR 2000\Assets

2) Open options.txt

3) Here, line 3, 4 and 5 are what we're looking for:

// Country codes
// 0 = UK, 1= UKZombie, 2 = FRE, 3 = GER, 4 = IT, 5 = SP, 6 = US, 7 = USzombie
COUNTRY 0

Modify the number on line 5 (COUNTRY 0) to change the language.

Save, launch the game and enjoy!

Be aware that it only change the text, not the voices.

Also, savegames saved with a different language are not detected.