Hello, South Park is completely region free, but if you live in a country which is 'censored' then Steam will not usually let you activate, since we're selling the 'uncensored' version.
In order to activate the key, you must activate the game using an IP address which is located in a country that is 'uncensored' such as the UK.
To do this, you can use a VPN.
You can do it within 2 minutes by following THESE INSTRUCTIONS:
Follow this tutorial, but instead of selecting a 'russian' vpn, select a 'united kingdom' vpn. You will be able to then activate the key.
Once activated, the game will function in all regions and languages.