Content is hidden due to not choosing the languages in the settings.

Making the language settings opted in by default should solve this initial struggle, opting out should be done manually. Putting check boxes next to the languages on there for selection should make things easier too.

This has been an issue long before the guy in the linked post reported it. ~~I've had to face this same issue on other instances as well so it's obviously a Lemmy issue and not instance specific.~~

Please understand that these sorts of small but very visible troubles are what keep people away from this platform. You sign up and go to any community only to see no posts or only those not tagged with a language (You could even miss those if you don't choose "Undetermined"). Any new user would be confused "Are there no user at all?" "Is this community banned?" "Is this instance defederated?" Each of these doubts would just push any new users out.

Edit: This PSA was over 9 months ago now.

Edit 2: It seems to be instance specific, lemmy.world and lemm.ee both have this issue as far as I've seen.

Edit 3: Thank you everyone for your help.