You can edit the UserDictionary.lex file (located in the message store) in
Notepad and add words to it. If you can find a Portuguese (BR) list of
words, then you can paste them into that dictionary.
Not so much a bug, as a deliberate omission. I don't think the
development team had enough time to test many different languages.
Anyway, there is a free Vista email program that supports
Brazilian Portuguese: http://www.mozilla.com/en-US/thunderbird/all.html