Here's my two cents on the matter.
As you know, since you linked to it, the policy is that questions and answers are asked/answered in English. There are StackOverflows in other languages that should be used for those who don't know, or don't want to use English.
Comments are "second-class citizens" on the stack exchange sites, so sometimes they do have policies that're slightly different... but IMO there's no reason to comment in a different language.
When you mouse over the "add comment" link it says:
ask author for clarification about this post
And that's what they should be used for. Now if you felt it needed to be clarified, then the odds are good that others do too and if that's the case you need to be writing those comments in the "native" language of the site, which is English.
If you're not adding a comment asking for clarification, then it's possible it's off topic and it's fair game to be flagged and deleted by a mod. If I post on someone's Q or A "hey, I'm from Ohio too, what city are you in?", well.. that should be deleted because it doesn't add anything of use. So if you feel the need to "talk" to someone in your native (non-english) language, consider the content of what you're going to say... why do you feel that this person, and no others, need to read that comment? Maybe it would be better if that comment was made in a private chat.