System.Globalization.CultureInfo.CurrentCulture seems to always indicate the "en-US" culture, which makes correct parsing and display of dates, currencies, etc. difficult. I believe this is because Unity is using the reference implementation of the Mono CLR, namely https://github.com/mono/mono/blob/master/eglib/src/gmisc-win32.c, in which g_win32_getlocale explicitly returns "en-US".
I just added a reply to this thread -- http://forum.unity3d.com/threads/5452-Getting-user-s-language-preference?p=1076329 -- in which I explain a workaround for the Unity Editor and Windows standalone builds. Unfortunately it doesn't work in the web player because I can't use DllImport to call into kernel32.dll.