Hi Vale2,
Yes, it should be fairly easy to do so:
You can choose to install both languages during the install (i think)
If you have already installed it:
Right click avast! tray icon --> program settings --> language tab --> click 'To install additional languages, click here'
You will then be prompted to 'change' the installation, (second image) --> scroll down to portuguese and then click the relevant boxes with a red cross in it --> it will change to a tick--> click next.
The language will then be listed in the languages section of the settings (similar to my image)
-Scott-
edit: Oops, Tech posted also...well either way works...