This is quite easy in Mendix, here is some Mendix documentation specifying how this can be done:
In your pages you can create something for the user to change languages, like a dropdown or flag pictures. When a user selects these, you can have a microflow change the language which is associated to the user.
I do believe you need some extra functionality in order to change the labels/text in real-time on the screen for user. Otherwise the change will only be visible after new login or hard refresh (F5) of the page.
Some more details about this can be found in https://forum.mendix.com/link/questions/87571