Open redirect in "Language change".

Summary

This report turned out to equate to three different issues:

• The language chooser not stripping excess slashes correctly.
• Our web server responding to internal requests from other "FQDNs", which caused the unwanted redirect.
• CloudFlare returning error 523 when attempting to access one of those URLs.

We awarded a single bounty for the first two issues since they require each other to do anything possibly unwanted/malicious. As the last issue (the incorrect CloudFlare error pages) was solely at the CloudFlare level (and outside any of our control), it is not eligible for a bounty on its own.