UI view language reverse proxy

Description

Steps to reproduce

1. Set up a CMS site listening on port 8000.
2. Set up a reverse proxy listening on port 80 and proxying to 8000.
3. Make sure the reverse proxy URL is set as site URL and as a host in site definition in admin mode.
4. Navigate to a page in the Episerver CMS that has multiple languages.
5. In the Page Settings edit area, select a different language.
6. The page reloads but the content is still in English.