Set the wiki to multiple lingual
Create a page with language "fr" and "en"
Create a second page with language "en"
Visit the first page and force the language to fr (with language=fr in the URL /xwiki/bin/edit/Space/Page1?editor=wiki&language=fr)
Edit the second page without anything in the URL ( /xwiki/bin/edit/Space/Page2?editor=wiki)
Modify and save
This will create a page in language "fr" for Page2
The reason is that the cookie setting is "fr" and is used to create the new page. We should always ignore the cookie setting if the language was not explicitely set using the "language" param and the page does not exist in this language.