function GetWindowsLanguage: string;
var
WinLanguage: array [0..50] of char;
begin
VerLanguageName(GetSystemDefaultLangID, WinLanguage, 50);
Result := StrPas(WinLanguage);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage(GetWindowsLanguage);
end;

{********************************************************************}
{ another code:
{********************************************************************}

function GetWindowsLanguage1(LCTYPE: LCTYPE {type of information}): string;
var
Buffer : PChar;
Size : integer;
begin
Size := GetLocaleInfo (LOCALE_USER_DEFAULT, LCType, nil, 0);
GetMem(Buffer, Size);
try
GetLocaleInfo (LOCALE_USER_DEFAULT, LCTYPE, Buffer, Size);
Result := string(Buffer);
finally
FreeMem(Buffer);
end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
Caption := GetWindowsLanguage1(LOCALE_SABBREVLANGNAME);
(*
LOCALE_ILANGUAGE { language id }
LOCALE_SLANGUAGE { localized name of language }
LOCALE_SENGLANGUAGE { English name of language }
LOCALE_SABBREVLANGNAME { abbreviated language name }
LOCALE_SNATIVELANGNAME { native name of language }
*)
end;