Updating default language in XWiki Preferences does not update user interface language

When setting up let's say a french language xwiki, I go to the XWiki Preferences and set

MutilLingual : Yes
Languages : fr
Default Language : fr

If I happened to have been manipulating the wiki before that, there is a language cookie set to en. This cookie stays set to en and causes the user interface to continue to be displayed in en. If I delete the cookie manually on my browser, then ui shows up correctly in french. This also causes problems when creating a new page with the Create Panel because the initial language version will be en instead of fr.