If you download and install gootool there is a translation option there which you can choose what language you want.I don't know the details but you should find Portuguese there.

Well, there's already a range of different languages, and while you can select them in GooTool they aren't all actually installed in World of Goo yet.

DavidC Maks made a small online application that can convert translations on the WoG Translation Wiki to a usable format, however you'd need to download the text.xml.bin file containing your languages, replace it with the original (preferably in your custom WoG installation), and then select the appropriate language in GooTool as naveed mentioned above.

For simplicity though, I can make a goomod that contains a modified text.xml.bin which has all the languages in it, so that it takes no more work than just installing the goomod and changing the language in GooTool. I'm going out right now but when I come back I'll make and upload the goomod.