function WhichLanguage:string;
var 
  ID: LangID;
  Language: array [0..100] of char;
begin
  ID := GetSystemDefaultLangID;
  VerLanguageName(ID, Language, 100);
  Result := string(Language);
end;

...

procedure TForm1.Button1Click(Sender: TObject);
begin
  Edit1.Text := WhichLanguage;
end;