This isn’t possible right now because Micro.blog manages the frontmatter for you. If all your posts are in Portuguese, you can use a custom theme to set the language everywhere, but not on a per-post basis.
This is something we need to fix. We’ve talked about having some kind of interface for adding custom fields to the frontmatter.
Another work-around would be to use categories like “English” and “Portuguese”. Then you could edit the templates in your theme to show the flag if the category was set. Let us know if you want to try that and someone here (or me) can offer some tips.