Language doesn't stick

I have a SE K610i. When I update, the Danish language file works fine at first, but then when I turn the phone off and on again, it's back to English menues. The text-language seems to work fine, but in the Phone-language, I can only choose English. Before the first reboot I can choose Danish, English, and a few more.

Why does it do that?

SE K610i
XS++ 3.1
K610_R1JG001_FS_EUROPE_4_RED52.fbn
K610_R1JG001_MAIN_GENERIC_TH_RED52.mbn
own_custpack (tried with and without)

Post your Setool identify log or XS++ log. Did you used the right custom pack file for EUROPE_4?
Use this: HB1-06_Generic_CDA102568_13_R3A_GENERIC_EUROPE_4

1st time did you use cust pack???

open the custpack in notepad find these lines
<allowed-language>en</allowed-language>
<allowed-language>fr</allowed-language>
<allowed-language>de</allowed-language>
<allowed-language>tr</allowed-language>
<allowed-language>st</allowed-language>
<allowed-language>zu</allowed-language>
and then add in the danish language (i can't remember the code for danish)

most likely customization pack was not for your FS version. this is the origin of all your trouble.