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;