Sorry, I can't immediately see what is going on.
I suggest that to try to work out what is happening, you start with the simplest possible question, then add a bit at a time until it breaks.
Is multilang avialable on https://qa.moodledemo.net/? Yes, 'Multi-language content' is enabled there. So, test there as well as your own site, just to check what you are seeing is consistent.
That misses the point. Some of the work that is sitting there not yet reviewed is work that was not integrated into 5.1, and got held over to the 5.2 cycle (if not worse).
Does Moodle HQ value community contributions or not? At the last MoodleMoot Global, all the senior mangers stood on stage and talked about how much they valued community contributions. But, persistently, HQ under-resources the integration team. There was a great effort made in September and October last year, which finally got the queue to under 100, but then it was just allowed to grow to over 150 again. (Data from Integration exposted.)
You need an integration team that, on average, itegrates more issues each week than on average are submitted to integration. HQ have been monitoring the key metrics on that for years (as linked above) the metrics show that the team is not big enough, and the mount of resourcing is not changed. Why?