Entire communities are empty and some post comments can't be seen when I'm logged in as an admin.
When I'm logged out I can see everything as it's supposed to be. However, when I'm logged in as one of the admins of our instance, then there are entire communities that are empty and some post comments are not visible.
In your personal user settings for language, do you have "undetermined" unselected? When I unselected it, I couldn't see any posts that were made before v0.17/didn't pick language.
I'll soon add a warning to lemmy ui about this... that when you deselect undetermined, you really are only going to see things explicitly marked in those languages.
And maybe point out that this will impact content from older versions of Lemmy, Mastodon and any other source that doesn't enforce language tags. A lot of content falls through the cracks when you don't include it!
And there can't ever be a guarantee that fediverse content will include a language. I think selecting a language should only filter out content with an explicitly set language that doesn't match but allow unspecified-language content through.