Hmm, I went ahead and followed the instructions that I had written, and I agree that things seem to be completely broken (particularly, the “default locale” is not applied when you log into the Reference Application).
I had already written a ticket about improving the usability of this:
Now I have added another ticket about fixing the bug:
Both of these tickets are good introductory tickets for any developer.
So, if you go to the legacy UI, and change the language in the footer, it should remain translated after you navigate to the next page. For example I see this when I change the language to French there, and click on Home: