The title on the second page is kind of bothering me too (the page where you had to enter the username/password to go on). It says:
afg bbb bai bab adc aig bba baa adc ahd bab bab aef
If you use a = 0, b = 1, etc., then you get a numbers, which in ASCII again go to:
8ole Vnd Iee-
That's 8oLe ... iee (making sure of no discrepancies on i's and L's). I would say this is just gibberish, but the fact that there's 'adc' twice (which is 032, or a space) makes me think that it's supposed to mean something. "Bole und lee"? Maybe that means something in another language.
Next, I'm going to clear up what I posted above:
Here is the translation of the title of the page:
On the Zuiderzee
That old fishing sea,
The Faeiries stole,
The boys of Marken
Next, here is what was commented out in the HTML source:
The old name of the site of the boy's disappearance is the name of the place you must go.
There is some text "hidden" beneath the picture, highlight it and translate and you get:
hebban olla vogala nestas hagunnan
hinase hi(c) (e)nda thu
uu(at) unbida(n) (uu)e nu
Lastly for the translations, here is what I got from Captain_HumpYourFace's 3 lines:
1590521471 fourth word add s
0060254920 fifth word
I'm not sure what those mean. Captain_HumpYourFace, if you can type out more of that stuff and post it here, I or whoever can translate it... I wrote a small python script to "automate" the translation, because I couldn't find anywhere online offhand that did multiple numbers at once.
The last thing worth mentioning is that in the popup that says "De jongen zal alleen sterven." the date that it puts in front is whatever the current date is on your system, according to the source.
That's all I've got for now, maybe more later.