Thank you for that hint, kronovan. I couldn't find anything, though. Could you provide a page number, please?
My bad, I was recalling my adds to the canon from my Duchy of Oriente campaign notes. There are Italian and Spanish speakers in the Duchy of Oriente, so I wrote in that people of Portuguese descent can be found in the Suva Archipelago of planet Fletcher, living among peoples of Italian and Spanish descent. I sometimes tend to get carried away with details in my campaigns, to the point that they get recalled as actual canon.

While browsing my Federated Suns Handbook last night for some campaign fodder, I came across a reference to Portuguese. According to a brief entry on page 135 in the Society and Culture chapter, Portuguese speaking peoples are living in some of the worlds along the Capellan March.