Doesn't anyone think that it is very rude to comment in someone else's language unless it is not understandable - just plain RUDE?

Sometimes. For example I'd probably consider it slightly rude to reply to this with "s/comment in/comment on/". That said it is a borderline case since 'comment in someone else's language' actually means something (unintended) and so I needed to read your comment twice then look up the context before I could guess what you actually meant to say.

In the case of top level posts in main corrections are entirely appropriate. A certain standard is expected for main level posts. If that standard is not met then the alternative to polite correction is a silent downvote - many people prefer the correction.

If someone wants help with language they can ask.

If someone is particularly sensitive to correction they probably shouldn't make top posts - or, preferably they can ask someone to proofread for them before they post. This is actually what many people do anyway even if they have no language difficulties whatsoever. In fact there are people who have volunteered to proofread drafts for others as their way to contribute.

Language is a tool not a weapon.

Typo and grammar corrections don't hurt as much as having your arm hacked off by a claymore either. I certainly don't consider Solvent's comment an attack.