How to allow the user to change the language?
You can find a detailed answer in step 5 of this guide: 5 Steps to Build a Drupal 8 Multi-lingual Site.

Basically, this is what you need to do:


Go to Admin > Structure > Blocks
Click "Place blocks" on the desired region
Click "Place block" next to the "Language Switcher" block
Click "Save block"
Go to a translated node and the Language Switcher will be working




As a sidenote mentioned in the comments of that guide: "You also need to have the url method enabled in the detection and selection page."