The Swedish dictionary is provided by The Swedish Academy (Svenska Akademien).
The Dutch dictionary is proprietary, created by TaalTik, using the OpenTaal dictionary as base material. You can get more info about the words in the list from wordfeud.taaltik.nl.
Wordfeud uses Dansk Sprognævns Retskrivningsordbog, 4. udgave, 2012, inkl. 8. digitale oplag, 2017. It also includes the latest updates from 2021.
Wordfeud uses "NSF-ordlisten", made by the Norwegian Scrabble Association (Norsk Scrabbleforbund).
The Portuguese dictionary is proprietary, created by spellonit.com. For more information and / or feedback, see http://spellonit.com/support/por/