Pretty sure that a US model firmware won't include Portuguese. So read the descriptions of the custom ROMs in the android development subforum, looking for 900F based ROMs that have Portuguese support. When you go through the new user setup steps, it will ask you what language you wish to use. You just need to locate a ROM that has Portuguese support.
In the alternative, do a Google search for 900F firmware that Portuguese carriers use. Then flash that to your phone. There is a risk of making your phone unbootable or soft brick when flashing firmware intended for a different variant model /carrrier. To avoid that, you should also search for a PIT (partition information file) that matches the new firmware and flash both at the same time.