How to use _user_mail_notify with a defined language
I finally understood and overcome the issue, in order to send emails in a different (enabled) language multilingual features must be enabled for the emails.

To enable the multilingual features there are just few steps:


The module "User mail translation" has to be enabled
From the Variable configuration (admin/config/regional/i18n/variable) you have to select the tab "User emails" and then check the email types you want available in a different language
From the account setting page (admin/config/people/accounts) then you select the language (in the very first box of the page) and add the translation for the related email templates (just scroll down)


After this 3 steps you will be able to send emails in all the enabled (and translated) languages in your site.