suddenly most of the system interface switched to my second system language, which has never happened before.
I noticed that this occurs when I disable Developer options. It's a reliable way to reproduce the bug for me. But not sure we're talking about the same bugs.
Sometimes difficult to ascertain whether or not something was caused by having Developer options enabled. Here's an example of a bug that triggers when the user disables Developer options. LOL.
I'm guessing that disabling Developer options somehow triggers a resort of the preferred system languages list, and that languages that do not have AOSP translations – but that are still selectable from the Settings UI as "non-system" languages – get downprioritized due to… some logic. It's probably refreshing that UI anyway because it needs to get rid of the pseudo-languages that are only displayed when Developer options are enabled. Anyway, seems pretty niche.
Edit: This is unlikely to be a regression, so kinda offtopic here.