I've already got it to work. I just used this topic: Flash K800 CID53 with K810 software CID53 (for begginers) - Top Sony Forums and followed steps 2 to 12 (with a DCU65 cable) and it worked.

For step 6 I used: W610_R6BC002_MAIN_GENERIC_NA_RED53
For step 7: W610_R6BC002_FS_EUROPE_3_RED53
And for step 8: HB1-06_Gen_CDA102568_12_R8A_GENERIC_EUROPE_3

Exact same firmware, exact same everything but with the Dutch language now :)