Some people have been asking which communities they should join first, so I posted some remote communities you can subscribe to on the sidebar on the homepage :)
There's also this universal community search tool you can try using. If you find a community, just copy its URL and paste it in /search to subscribe to it here. This just goes to show that while there might not be many local communities here on lemmy.one yet, the beauty of the fediverse means that doesn't matter!
I'm still hoping that this community grows here, but so far the only one I found for C is [email protected] (all the other programming languages seem to also have a community on programming.dev instance)
Atleast one person from your instance needs to subscribe to a community on another instance so that others can find it in the all communities list. If none have, you need to paste the real url into the search to find it.