Strings

add_filter('gettext', 'mycustom_func_1', 10, 2);
function mycustom_func($translated_text, $domain){
    if($translated_text == 'Username or Email'){
        $translated_text = 'Username or Email in Polish ';
    }
    return $translated_text;
}

Visto en https://toolset.com/forums/topic/cant-find-couple-string-to-translate/