After more investigation, it looks like older users had those categories muted, which seems to mean that those categories can’t be found by most users.
The reason that some foreign language posts were showing on the home page is that I may have read enough of those topics to unmute them.
The reason that I could only see the Nederlands category and not the other non-English sections is that the muting must have happened before we added that category. The other non-English categories were older. I’m guessing that the test user could see all of them, because the test user may have been created after the change.
In our case, the categories should be discoverable for all users. The non-English sections aren’t as active as the English sections, but there are VIP people who post in there, and some enthusiasts will translate them with the browser even if they don’t reply.
It isn’t a long-term solution, but for now it’s better if all users have those categories unmuted. I’ll try to write a plugin that lets people mute categories with one click while viewing any topic in that category and/or maybe a list of checkboxes for those categories on the settings page.
Is there a quick way that we could unmute all of those non-English categories for all users?
Also, is there anyone else out there who has foreign language sections? What is your solution?