That's definitely on my Lemmy wishlist -- some way to create merged communities. As it is now, there's too much potential for fragmentation across instances.
As of now, I just subscribe to whichever instance's community is the most active / has the most subscribers, which feels sub-optimal.
I think the problem is that you have to have at least some understanding of how the federation aspect functions to make sense of Lemmy. I'd love to contribute to their UX, there's a lot of room for improvement IMO.
Same problem as with any social network migration: the network effect.
Sometimes the largest equivalent Lemmy community for a subreddit is tiny, or worse, nonexistent. Sure, you can go create one, but unless you can also convince other people to join with you, it won't be much fun.
Two other "easy" bits for Japanese: