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;