Ugh. Starts off logical, but at the end there is no way to do this other than mass guessing and use of temporaries. Some parts of it can be guessed from context, but others are completely inscrutable and you just have to try every possible combination of squares. Don't waste your time. (The dutch translates to "Find the good way" -- no help there)
9 point diagonal from the upper left corner, 5 point diagonal from the upper right corner (then bounce back until you hit the 11), 12 point diagonal from r1c15, 6 pt diag from r9c13 to the 5 in c8.