I don't think it's a major offense to reply in your own language but since most of Lemmy is English speaking I try to respect the spaces that are clearly meant for something else.

I like to translate what I'm posting to whatever language the community is using. If I mention I'm using a translator the OP or another commenter will reply in English if they feel comfortable. !bubatzgartenclub@lemmy.world is one that comes to mind where this has happened in the past.