Translation:It is traditional to get married in church.
How about, "It is tradition (or traditional) to get married in a church." Why not? In American English "in church" is a bit different than "in a church." The former connotes "during a church service" while the later denotes a location. There are things we would never do "in church" that we have no problem doing "in a church."
I'd say no - the Swedish phrase does not mean in a church, either, but rather as opposed to having a civil/secular marriage. It's not possible to translate perfectly etiher way, though, so I can see the argument for acceping "in a church" as you suggest as well.
Yes, but as noted above, that is not what the Swedish sentence means. In essence, the sentence says that it's traditional to get married in a religious ceremony - not necessarily physically in a church, although that would obviously be the case for most religious weddings.