Polish state dinners have a black tie dress code these days. That means evening gowns for the ladies, but no orders. Some Queens wear tiaras for such a dress code, like QEII and Maxima. Silvia used to as well, but didn't at the recent visit to Lithuania. Tbh, I don't think that Mathilde will wear a tiara, but she'll surely look stunning in a beautiful gown.
Edit: According to a post by EmmaNL just posted she did wear a tiara. But it can't yet be seen on the pics.
http://www.theroyalforums.com/forum...for-upcoming-events-25440-94.html#post1828916