There are some modules available to allow users to select a language, for instance the Language Selector. Is that what you're looking for?
It is as Marius mentioned, you will need to change the language of the user by changing the association user_language and refreshing the page.
An example of such an implementation is to create a button that retrieves the required language, and set user_language association of the current user to the retrieved language and commit.
After that, you can open a popup page that has a URL redirector widget https://marketplace.mendix.com/link/component/118716. In the widget you can just put the URL of your application to refresh it, and the reflect the language change.
Note: You can pass a value to the URL redirector, so you can actually pass the URL that you want to redirect to the widget. This is great because you won’t need to hardcode the URL in the widget.
This documentation on language should help: https://docs.mendix.com/howto/collaboration-requirements-management/translate-your-app-content
The user's language is stored in the User_Language association, so you can add a dropdown to a screen where a user can select a new language through that association.
Do note that you might need to refresh the page in order for language changes to take effect.
Add a language that you want, from Language setting.