Translation:It is a tradition to get married in church.
"Det är tradition" is a phrasal expression corresponding to "It is traditional" (in my opinion). Even "It is the tradition to get married in church" sounds inelegant to me.
I don't think "a tradition" is necessarily wrong, but I definitely do agree that "traditional" should be added. We have a quite a few reported variations using that. I'll be entering them right away.
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.
Is the sig mandatory in this phrase, or can "Det är tradition att gifta i kyrkan" be accepted too?