good choice since nokia 520 rm-915 variant has wcdma band 2100 mhz so you can use 3g in the netherlands. Nokia 521 rm-917 lacks this, so you would have get gsm only in europe.
To unlock you can try to enter ##7820# - after some seconds you can enter the 20-digits code.
One thing as to not confuse people the 521 is a t-mobile phone that relies on the 1700/ 2100 band... So it does have the 2100 band... If anything the at&t 520 doesn't have that band.