Hi Joel,
please try using "pt-br" instead.
Also, as Neill said, you should use "Change site" or restart your app to refresh the strings in the app. This data is only retrieved when the user logs in in the site or the app is restarted.
If that doesn't fix the problem, please send us a test account to mobile@moodle.com so we can take a look.
Cheers,
Dani