So if you want to force the default path prefix in all links. The following trick can do for you. Open the /includes/language.inc and edit the language_url_rewrite() function as follow.

You have to comment line below as shown

syntax:

case LANGUAGE_NEGOTIATION_PATH_DEFAULT:

//comment line below

/* $default = language_default();
if ($options['language']->language == $default->language) {
break;
}*/
// Intentionally no break here.