You should use GetSystemLayoutList function to get identifiers of all system languages and use VerLanguagename function to convert these IDs to locales.
procedure TForm1.Button1Click(Sender: TObject);
var
Count, i: Integer;
MyLang: PChar;
Layouts: array [0..16] of Integer;
const
Size: Integer = 250;
begin
GetMem(MyLang, Size);
Count:=GetKeyboardLayoutList(16, Layouts);
Memo1.Lines.Clear;
for i:=0 to Count-1 do
begin
VerLanguageName(Layouts[i], MyLang, Size);
Memo1.Lines.Add(StrPas(MyLang));
end;
FreeMem(MyLang);
end;