Hi Giuseppe, Based on your question I assume you have an anonymous userrole in your project. Mendix is a single page application, so it isn't possible have different index.html pages for each language. The language is associated to the user and based on that the client/browser is using that language.
It is possible to use the deeplink module to publish different links for each language. The link would be: https://www.example.org/link/language/en (1 deeplink for multiple languages) or https://www.example.org/link/en (a deeplink per language). The hardest part is to attach the language to the user and reload the page again. The language is used when the client is loaded and after that the deeplink is triggered so the language would not change. You could use the URLRedirector widget to reload the page with the new language.