Introduce Textpattern_L10n_Locale::getLanguageLocale().

Returns the locale for the given language code. Basically
the current getlocale() but without setting.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5626 2fea6a4d-a838-0410-917b-93a53c48e9d2