Ok Pia, let's start with some corrections, I can see that we have a long way ahead...
decision ---> resolution
deregister you ---> withdraw your registration
migrated ---> migrant (or inmigrant?

Is the person a foreigner living in Sweden or a Swedish leaving the country?)
This paragraph should be changed a little.
"According to 20 § folkbokföringlagen (1991:481) shall a person that can be assumed to regular spend one's sleep outside the country during at least one year be deregister from the national registration as migrated."
into:
"According to 20 § Legislation on National Registration (1991:481), a person who is assumed to regularly spend his sleep outside the country during at least one year, shall have his registration as an inmigrant withdrawn."
And the last one into:
"According to your own report "Notification Moving Abroad". The Treasury considers that there is a reason to withdraw you from the national registration."
This is quite confusing, so let me know if I am wrong, OK?