I'd link between the two, but I'd avoid flags for the reasons you probably know... is Portuguese the Brazilian or Portugal flag? Is Taiwan a country, or a part of the PRC?

One thing we do a lot with a site that I work on for a client is to cross link where it makes sense --- like to an article that is only available in the other language, usually with a note after the link e.g. something like "Catálogo(inglés)". Don't know if this will work for you --- is it common for people to speak both Dutch and Portuguese?

HTH,
Mark