There has been discussion on various ways to improve multilingual usage of BIP39. https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-January/015500.html

My takeaway is that we should be implementing a standard way to convert any words into a hex string, regardless of language or character encoding. In that discussion there was some notion of "who needs anything but English words anyway?" rhetoric, so I'm happy to see yet again that there exists an obvious need to support all languages for a standard in wallet seed generation.

I realise that doesn't help you today considering there is no such multilingual standard or official Portuguese wordlist, but I wanted to express that this is an ongoing topic of discussion and it's clear we need to start choosing/codifying a standard in this regard.

That said, here's somebody who has started a Portuguese wordlist so maybe you can start from there. https://github.com/bitpay/bitcore-mnemonic/issues/52

Regards,

AJ