Maybe you can have the active menu (at the moment it is just the hover) item a red background as well?
Can the footer be outside the white box with the writing in white.
The certificaten page can you change it so that it is one row of images going down left hand side of page, and next to it on the right the text
On the contact page, I see a polish or some other language, can you change to dutch:) Wyświetl większą mapę
Can you add some margin on the top of site? (so you can see a part of the background)
Additionally, can you have small rounded borders around the site?
(css border-radius will do, don't worry about css2)
Is it possible on the projecten page to have each photo titled on the image Project 1, Project 2 with a semi transparent black box behind the writing but on top of the image?