Changing language on email confirmation page takes user back to registration

### Steps to reproduce

1. Register a Matrix account
2. Click the verification link in the email
3. On the verification page, change language

### What happened?

The page refreshed and took me back to the registration page instead of showing me the message in another language

### What did you expect?

Message is displayed in the correct language

### Operating system

Arch Linux

### Browser information

Chromium Version 92.0.4515.131 (Official Build)

### URL for webapp

develop.element.io

### Homeserver

matrix.org

### Have you submitted a rageshake?

No

Reproduce the problem on develop.element.io with the listed registration, verification-link, and language-change steps. Start by locating the verification page and its language-change flow; done means changing language keeps the user on the verification page and displays the message in the selected language.

Written by the indexing model from the issue text.