Is there any convention for interactions in Meta? Can I publish in English and in Portuguese?
The answer from French's meta is still completely in-line with what we expect on our Language sites, but I'd add that it also depends on what it is you're asking about.
Unlike Stack Overflow in Portuguese, this site is a mixed-language site. It is not purely Portuguese, and English questions are completely on-topic here. Therefore, it is kind of impractical to write things here on Meta in only Portuguese because:
Only the staff who can speak Portuguese will be able to read and understand them. While we do have some staff who are fluent in Portuguese, not everyone is.
You outcast the English speakers because they don't know what the rules are. For someone who is learning Portuguese, they might not be fluent enough in the language yet to understand some of the topics which have been brought up or understand why a question might not be acceptable here.
But these things can also apply in the opposite direction. It's just one of those things that happens on a language learning site where you just have to target two language audiences to make it work.
So back to the "it depends" - if you want to reach the maximum audience of the site and allow opinions from everyone, you're better off posting things on Meta in both languages. If you aren't fluent enough in English, that is fine - we also encourage users to translate any posts to English or Portuguese (depending on which language it was written) so that the entire audience of the site can understand the community and what is being discussed on Meta. This is a very common convention that has been adopted on our Language sites (the link is an example of this occurring).
Obviously not every single issue needs the attention of the entire community, but large site decisions and frequently asked questions which concern the entire community and come up frequently should be available to everyone in both languages.