How can I switch back to US server (Left 4 Dead 2)?
You can change the download region for Steam on your PC:Steam/Settings/Download+Cloud/Download Region and set it to United States.
Alternatively you can open game's console menu and enter the command openserverbroswser to view available games and their IP addresses. Then, still using the console menu, run the command mm_dedicated_force_servers [IP] substituting the [IP] with the address of the server you want to play on (no box braces []).
Both of these methods of changing your server are documented on this Steam discussion