Fediverse

I don't do it, but if I did, I would consider apologetically offering the machine translation inline with my post. Why put the burden on them to do it if you want it to be read?

いや、大丈夫だよ。

Honestly though, I think it depends on the context. I think it's generally OK on open multilingual platforms especially with mixed audiences.

I see lots of English comments on Japanese vocaloid videos, for example, and I think most content creators enjoy having fans from abroad.

I've had more conversations than I can count with people I would never be able to talk to in person, all using our own native languages.

The original posts are in English, people comment in their native language, and I use a translator, then respond in my own language. Is the translator perfect? No! Neither is theirs.

With the way most translators I've used work, it's easier for the non-native speaker to try translating, since the translator might try and use different words that entirely change the meaning, but likely list possible alternatives. A native e speaker will understand the alternatives while a non-native speaker probably won't.

That's my thought process anyway.

Never had anyone who wasn't pearl-clutching or virtue-signaling complain about it. And I've had tons of conversations with people I'd never have talked to otherwise.

I'm not sure about other places, but in mod comments on Nexus it's fairly standard to just reply in your native language and have the other person translate.

You'll often see discussions with one half in English and the other in Chinese, for example.

Generally Japanese posters enjoy knowing they have fans overseas! And it's better to type what you intend than attempt to type in a language you cannot speak. It doesn't look rude at all though~

I would be a little careful of words with opposite meanings though or idioms. Like "that song is sick" or "that's tight". Be direct with your post so the auto translator can pick it up properly.

I often reply under Japanese posts, and I always assume users will use a translator as I do, but maybe in the context of a Japanese this may look rude?

Can't speak for others (obviously, as this is about individual etiquette perceptions) but I would consider it to be polite to only enter conversations with unknown parties in languages that the parties have shown to be capable of speaking and understanding.
Using a new language entering a conversation would therefore signal either familiarity ("I know they understand me") or rudeness ("I don't care if they understand me") to me, I suppose.

I guess, you could try to reply in Esperanto,it's most non offensive language I know.

If you answer in the language you know best, it'll be easier to others to understand or translate, especially if it's English.

You could translate your message to match the language of the comment, but if you don't know the language, how can you know if it conveys your message correctly?

Overall, I'd say it depends on the specific community. If you try to inject yourself into a conversation in a Japanese language community, it may indeed come off as rude or ignorant.

The best solution may be to post in both languages?

Hmmm... I thought it would be rude, but considering the consensus here, people speaking other languages should just respond using their languages to English comments and posts. There are way more non-English speaking people on the planet than English speakers. It would make the fediverse truly international if people did what you did!

Thanks for possibly starting a movement :)

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.

Okay where do you even see Japanese posts?