I believe the second screenshot gives you the answer, there is an option to force https in the discourse parameters/security tab
In your browser console you can see: Chargement du contenu mixte d’affichage « http://forum.mystermc.pl/uploads/default/original/1X/61433630046b732899eab0721da7dcb839bd7c5e.png » (non sécurisé) sur une page sécurisée
(sorry for the french )
As for the translation problem, it might have to wait a little bit because off the transifex/crowdin swtich, I saw a post dedicated to polish translation… here !
)