On build WW_35.0604.0404.140, I'm experiencing a bug that was previously reported against other versions of Android. Unclear if this was fixed--I couldn't find an AOSP bug linked anywhere, but the Pixel thread suggests it may have been fixed?
Broadly, having secondary languages enabled means that sometimes, random subsets of the OS UI and third-party apps will swap to using the secondary language. Unclear what causes this, but it seems changing language settings or switching keyboards may trigger it. It sometimes resolves on its own, and sometimes requires a reboot to fix.
In the attached screenshots I've included my language settings, with English as primary, Russian as secondary, and Kazakh as tertiary. The main settings section is in English still, but the drop down menu (which is part of Microsoft Launcher technically, I guess---I do not use the standard launcher) is in Russian. I've also seen the reboot menu and my third-party alarm (Sleep As Android) stuck in Russian.
English is definitely available for these UI elements and apps because, well, it's English, and also because I've seen them all display in English as expected.
https://old.reddit.com/r/bugs/comments/1bqib49/android_app_keeps_switchen_language_from_english/
https://old.reddit.com/r/GooglePixel/comments/1izptdv/randomly_multiple_languages_in_the_ui/
https://support.google.com/pixelphone/thread/324893944?hl=en&msgid=327964047
