I installed the english version of Visual Studio 2013. The GUI is in English but compiler errors are in French. That's a nightmare when I want to Google an error...
How can I switch C# compiler output language to English ?
You'd have to switch your machine's system locale to English, Control Panel + Language.
That's a rather impactful change since it also changes the locale for every other program on your machine. One thing you can try (but I cannot verify) is to whack the localization file that the C# compiler uses for strings. On your machine it should be located in c:\windows\microsoft.net\framework\v4.0.30319\1036\cscui.dll. Rename the file so the C# compiler can't find it and is forced to fallback to, hopefully, English. Btw, I guessed at 1036, there are lots of French locales. Locale IDs are listed here.