If such setting is implemented, please don't limit it to a single language. Many people knows multiple ones while still wanting to filter out all the other languages they are not familiar with.

I think something like that it is what happened. I found this image that has been posted in the fediverse trying to explain how things are visible.

In a similar situation, I'm currently using Mastodon (mastodon.social) and Firefish (calckey.social) following the same hashtags. As calckey.social is a smaller instance, there are many post not showing in Firefish. I tested this by checking the profile of a user who is in a completely different instance. Mastodon.social was capable of showing the most recent post of that user but calckey.social was still not aware of that post, as it might be the case that nobody from my Firefish instance has interacted with such post nor looked for it directly.

That sounds reasonable. What about if I use rich text in my posts? Will Mastodon try to show something or it won't even recognize the post at all?

Yes. People are commenting on that here.

I just confirmed it. Yes, the visualization of a profile (the fediverse community located in lemmy.world in this case), will vary depending on the instance being used to access it. It's really a pity though. Things like that should be transparent.

IMHO, whenever the user searches explicitly for a profile/toot/post/whatever, all relevant data should be fetched as well.

That's not a compatibility table, that's a comparison table (literally from the table itself). I'm not looking for what Firefish can do and Mastodon can not, I'm looking for how are those features interpreted in Mastodon. I'm concerned about discoverability and access. Right now Mastodon has way more users than Firefish and I wouldn't like to post things only to end up being visible only by people with Firefish accounts.

@xepher@calckey.world, can you also point to me how you got to this thread from Firefish? In Mastodon, I searched for @fediverse@lemmy.world and in that profile, that account boosts all the answers taking place in this community.

In Firefish, I searched for the same string @fefiverse@lemmy.world, I got to the profile page of the fediverse community, but I don't get to see any post/boost made by that account.

I also don't think this is a proper solution. The proper solution needs to be at a client level and not depending on your server.

Just to give you an example, I'm on lift off app and your link opens in a browser which is annoying.

That sounds cool but I've just checked this thread from Mastodon and I was not able to see this particular comment.

It's still very confusing what federates and what doesn't. None of the upvotes in this thread are shown in Mastodon.

Edit: This is very weird but now, after answering your post, I'm capable of seeing your post in both, the web browser app for Mastodon and in Tusky for Android. It was not the case before.