Steps to reproduce:
- Install the Wiki Template from the Administration menu
- Create some wikis
- Set multilingual to "Yes" from the Administration for all the wikis (including the main one: xwiki)
- Navigate to any wiki and change its language by adding "?language=fr" (or any other language code) at the end of the address bar
When you visit any wiki you will notice that all have the language that you set in the 4th step.
Besides this bug, I've noticed another one. You can use "?language=fr" (or any other language code) and change the language of a wiki to whichever language you like, even if the language code isn't in the Supported languages section of that wiki.