If you look into the method CultureInfo.CreateSpecificCulture() in the mscorlib used by the WebPlayer (using ILSpy for instance) you'll see that it is marked as SecurityCritical (you can read more information about .Net security here and here)
[SecurityCritical]
public static CultureInfo CreateSpecificCulture(string name)
and, AFAIK, all code from your game is considered to be "transparent" so it is not allowed to call into "security critical" code.
Also, if you check this page the method is marked as not supported on WebPlayer.