Admin forms using AJAX are broken after beta3 update

Hello,

After updating to beta3 version, some AJAX behaviors in our back-office are broken.

We have 2 languages, english (default and admin language) and french.
When trying to edit a node using the french path /fr/node/xxx/edit, we get the following behavior :

• We want to remove a file
• When clicking on the remove button: status message appears saying the node is updated and the file is still in the node edit form.
• Clicking again on the remove button, the file disappear from the form
• Then, when clicking on the Save button, the node doesn't save and "The content on this page has either been modified by another user" error appears

This is the most critical example, but some other admin forms using AJAX are broken too.

Admin language is in used for the following paths :

admin_menu/*
file/ajax/*
system/ajax
system/ajax/*
field-collection/*/*/edit
field-collection/*/*/delete
field-collection/*/add/*/*
file/add
file/add/*
file/*/edit
file/*/usage
file/*/delete
media/*/edit/*
media/browser
media/browser/*
media/browser/*
media/ajax/*
node/*/edit
node/*/delete
node/*/revisions
node/*/revisions/*/revert
node/*/revisions/*/delete
node/add
node/add/*
user/*/shortcuts
admin
admin/*
batch
taxonomy/term/*/edit
node/*/translate
user/*/cancel
user/*/edit
user/*/edit/*
taxonomy/*/translate
taxonomy/*/translate/*
node/*/moderation
node/*/panelizer*
user/*/panelizer*
taxonomy/term/*/panelizer*

After some testing, I found the #2640246: This module breaks language in forms with AJAX callback issue introduced this bug. This commit prevents all AJAX requests on admin pages to get the right language and breaks the normal behavior.