function GetWindowsLanguage:string;
var WinLanguage:array [0..50] of Char;
begin
VerLanguageName(GetSystemDefaultLangID,WinLanguage,50);
Result:=StrPas(WinLanguage);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
Label1.Caption:=GetWindowsLanguage;
end;