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?