Title: Get system default language
Use GetSystemDefaultLangID function. It returns identifier of system language. And after that, use VerlanguageName procedure. It translates this identifier to PChar-string.
procedure TForm1.Button1Click(Sender: TObject);
var
Ident: Integer;
MyLang: PChar;
const
Size: Integer = 250;
begin
GetMem(MyLang, Size);
Ident:=GetSystemDefaultLangID;
VerLanguageName(Ident, MyLang, Size);
Label1.Caption:=StrPas(MyLang);
FreeMem(MyLang);
end;