The problem was reported in the plugin discussion by Éric Bugnet: https://moodle.org/plugins/view.php?plugin=mod_realtimequiz

When using Realtime Quiz in French, the first page (quiz launching) isn't working. Launch buttons and text are marked as undefined.

Following Éric Bugnet work around, I removed the BR tag from the following lang strings: teacherjoinquizinstruct and teacherstartinstruct. This was done locally. It instantly fixed the problem.

I checked the official translation in AMOS and found out that both tags were still there. I modified both string and submitted my request that was accepted in the following hours.

I went to another of my Moodle sites (I manage over 50 sites) and updated the language pack. It had no effect. So I made the change locally. I could then witness that the official change was done, the BR tags were no longer there in the strings. I simply copied and pasted both in the local pack and problem is solved.

Conclusion: the issue is related to the main lang pack and not to the BR tag. To verify, I added both tags in my local strings and the display is just fine.