Use what @EXreaction said `->core_path('language/' . $user->lang_name)`

As for the 

> we use add_lang() and ext_add_lang() for core and extension language files respectively

IIRC $ext_name is an empty string for core files, so you can use

    if (!$ext_name)
    {
        $user->add_lang($lang_file); 
    }
    else
    {
        $user->add_lang_ext($ext_name, $lang_file); 
    }