However, the English version has been substantially revised and consolidated for rstan 2.18.2. So, if you previously contributed to any of those translations, or can translate into any of those languages, or would like to translate the English version to any other language, please do so on the wiki page. You do not need to be an official Stan developer to do so. Thanks!
As long as the instructions are correct, I would say that it never hurts to have translations in more languages.
It’s done https://github.com/stan-dev/rstan/wiki/RStan-Getting-Started-(Português). Note that I just revised the translation, it was @maxbiostat that has done all the hard work.
It is good to mention that I created a Portuguese getting started manual to Stan a while ago: http://marcoinacio.com/downloads/apostila%20stan.zip maybe it could be mentioned somewhere on the website.