It seems clear the English speaking web has a preference for Bluesky. It would be interesting to know how much variation there is between users of other European languages. It seems to me the Germans are pretty active in the Fediverse, which makes sense considering a significant portion of them have been huge privacy nerds since the fall of the GDR.
Those reaction emojis ± count like discord vs upvote/downvote is like going from Grayscale to Color TV. Worlds Apart. Night & Day. Misskey is leading the way.
タイッツー is definitely taittsuu. Twitter doesn't seem to use a katakana spelling. Your proposal of "tsuittaa" would be ツイッター. Same katakana but different order. There would be no reason to read Japanese from right to left. Might be an intentional pun though.
Is it spam, or is it literally just people using the fediverse as intended, coincidentally in a language you don't speak? Folks who speak a language other than english aren't less welcome in the fediverse.
If it's an issue because it's drowning out all the stuff you can read and engage with, I think you can set a Mastodon account so you only see posts in a certain language. Other platforms may have similar settings
It's spam. Most Fediverse software like Mastodon and Pleroma support MRFs that protect against spam, as I understand it, but Misskey doesn't. And since Misskey users are mostly Japanese, so is that spam, as it's targeting that Japanese audience.
If it randomly mentions other users, and if it comes in such masses that Mastodon admins have to raise the shields and Fediblock the hell out of dozens of instances, then it's spam all right.
That said, the last spam wave was organised on Misskey again, but carried out by bots from Mastodon instances largely abandoned by their admins. At least partially, this was the case for the first big spam wave as well.