The Swedish literal translation of middle name is mellannamn, which is a surname of an ancestor - usually your mother's if she kept hers.
The correct translation of middle name is andranamn, which literally translates to second name(s), and it/they are part of your given names. The first given name is translated to tilltalsnamn
The correct fields should be (P for plurality, S for single):
English | Swedish
------------------------
Either:
First given name(S) | Tilltalsnamn(S)
Non surname middle names(P) | Andranamn(P)
Surname middle name(S) | Mellannamn(S)
Surname(S) | Efternamn(S)
Or:
First given name(S) | Tilltalsnamn(S)
Middle names(P) | Andranamn och mellannamn(P)
Surname(S) | Efternamn(S)
I think the second version is closest to what you currently have in your database.
Source:
https://sv.wikipedia.org/wiki/Andranamn