Does anyone know why this user can't see our Blahaj community when they are logged in?

The issue was me setting “undetermined” as my preferred language. Turns out, you must press “X” in the web ui right next to the preferred language option.

In the essence, preferred language may have “undetermined” and “null” (both displayed in the front-end as “Undetermined”) as a valid entry, while neither being the exact same value in back-end, but being the same for the front-end.