Misskey has more of it. In fact, the second biggest fediverse instance is Japanese, mostly filled with illustrator, comic artist, photographers, gamers, and other otaku culture enthusiast.

Before mastodon.social, the biggest fedi instance is also Japanese, but it was different instance run by corporation (now seems to be abandoned after being sold to another corpo).

Even if people are talking in English, it still can display cultural difference. Especially nowadays we get Singaporean English, Indian English, Asian English, etc.

For example, a word in English Asia have neutral meaning, but in American English it is a slur. Unfortunately a lot of Western people does not realize this and tried to “standardize” the language. People should learn contextual language instead of policing from their own cultural mindset. Especially, billingual or trilingual people often code-mixing language.

Using English is totally okay!

I did it all the time and we interacted just fine.

Using machine translation can lead to mistranslation, even your heartwarming comment can be interpreted as hostile.

Depends on the context, commenting in your native language is often totally okay.

Let’s say: a Japanese artist posting an art with Japanese caption, they would totally happy to receive comment from various language, displaying a cultural exchange.

This behaviour of native language comment is actually common in Asia and Africa, but not in Western countries…

Just be wary of joke or sarcasm that might interpreted as hate comment.

Not really.

In Asia, people often just comment in their own language. Though, English is preferable for easier translation. Unless some extreme nationalist, most people simply happy to interact with you.

Edit: this is more common in Facebook. One single post will have various languages. Chinese, Hindi, Arab, Spanish, Swahili, and so on just in a single post. Sometimes, you can say that different social media, different internet culture. Twitter-alike social media usually more uniform in terms of language.

Just remember that it could be misunderstood, especially with sarcasm or joke.
I’ve seen Japanese artist deleted their account because they mistaken a joke towards their art as hate comment.