The code below uses unit SysUtils to easily determine the Operating System's default language. The TLanguages class also contains other information for each entry - check the online help on this type.
uses
SysUtils, Dialogs;
procedure TForm1.FormCreate(Sender: TObject);
var
i : Integer;
begin
i := Languages.IndexOf(SysLocale.DefaultLCID);
ShowMessage(Languages.Name[i]);
end;