O do not know if I am supposed to use a plugin for that, but probably I do not. I think that there is a way to setup which .mo should be used according to the domain at the wp-config.php file. I do not think that wpml is a good solution for my case, because I want the back-end changing the language according the domain too, not only the texts. Actually the best approach would ba a way to do both, change the content, but also the .mo according to the domain. Because I don not want pictures in english appearing for other nationalities too.