Ideally, a site for the Portuguese language should be conducted in Portuguese with an English component to help those who are first-day beginners. But to say "You cannot speak Portuguese here" (oops, I mean "Você não pode falar português aqui") would go against everything we know about full-immersion learning.
Anyway, this has come up several times on other sites, so I'll just quote the rest here:
It would be unfortunate to preclude Portuguese speakers in a Portuguese Language proposal. On the contrary, studying a language has long been known to be best conducted in a full-immersion environment (i.e. in the target language only).
We've even considered (on the Stack Exchange Team) urging our language sites to become "full immersion." Its long been known that foreign-language classes are most effective when conducted in full immersion, but there would have to be a culture of users where struggling beginners are helped when posting their "broken" questions… to help flesh out their meaning and to edit their posts into a condition that is understandable.
There are a lot of challenges to supporting the users of full-immersion environment, so we've been dragging our heels about suggesting it at all. But we would support any language site that wanted to step up and do this the right way.
And consider the Portuguese speakers who are looking for the Portuguese-equivalent of our "English Language & Usage" site. They would become a tremendous asset to making your site great.
In all reality, we are quickly becoming a world wide site, and we're going to look back on this someday and laugh that we ever allowed (on these language sites) to use English at all.