I've been trying to change the language of the login page to Portuguese since yesterday but didn't even get close to it. Does anyone know how to do that?

In cases like this, it's useful if you tell us what you've been trying so far. I could suggest a few things, but if I don't even know what you've been doing, I might suggest things you already tried.

Also, is that login page local to your eSpace? I.e. can you change it?
I'm trying to translate the page "*******.outsystemscloud.com/users/" , I've tried searching the name "Users" on the application, but I couldn't change anything there (So I though that I would only be able to change that on a paid version of OutSystems). Then I tried looking to the Development of the OutSystems Platform, but I found nothing there that could help me on that.

The Users eSpace is a Platform eSpace, so you can't change it. There are some translation included, but funnily enough the only Portuguese is pt-BR. Also, I unfortunately don't know how to switch to those languages...
Oh :(

Thank you anyway.
Just one more thing, where did you find that there are translations included?   
If you go to Service Center, and locate the Users eSpace, you can download it. From Service Studio, open the downloaded .oml, say "Yes" to creating a clone and inspect the eSpace.