The front of the envelope is pretty usual for the Belgium Royals. They have they have a lot of different ones that they use, but this one is pretty common. The front features a Palace of Brussels underneath.
Inside is a typed card & a photo of Queen Mathilde.
The photo was taken at Castle of Laeken, the King & Queen's residence. The [Royal] Palace of Brussels is a working palace where they have their offices.
The card is the usual from Queen Mathilde.
The card reads:
Palais fo Brussels
Her Majesty The Queen thank you wholeheartedly for the wishes you have addressed to her on the occasion of her birthday.
The kind words you have expressed greatly her heart.