This is a tough situation. Swedish and English are both Germanic languages, so at least it’s easier to learn than a language that’s from a different family, but that said it’s really hard to learn a second language, especially if you only speak one langauge. Does your FI also speak French? If so, it might help him to learn a 3rd because at least he’s familiar with a few problems that come up with learning a language.
I had to learn FI’s language, and it was really, really, really hard but I put in a lot of work, I have to say. FIL doesn’t speak much English, so now they all speak to me in FI’s langauge and I can have conversations with them. But I totally get what you mean with parents: FIL has to speak English to mom and dad and he struggles a bit, but they manage. I’m guessing your parents live in Sweden though, so it’s probably not so often they meet up anyway? But I know, it must be very frustrating for you 🙁
What about a private tutor for FI? That might help him learn much faster; he doesn’t have to learn the entire language, but just a little more so he can follow along when people are talking? I found cue cards helped a lot too. Can you also maybe supplement a couple Swedish words here and there into when you speak to FI so he can just get a couple more in his vocab?
Also, for the wedding, can you enlist any bilingual friends that can help with the translating load? Sit a couple at each table of visitors from Sweden to make their load a little easier, or try mixing 1/2 Swedish tables with Canadian guests and let them make their own conversation – and they will, don’t worry! FIL made a whole speach in German and the best man translated for everyone! Also, you can make up the menus in both languages, and perhaps try to encourage mom to speak a bit of English before the wedding, just to help practice for when she comes? We even had the ceremony in both langauges, so everyone could follow along; can your priest/celebrant do that, or can you at least have a friend do a reading in Swedish?
You will be fine, don’t worry. Your wedding doesn’t need a specific langauge to be special, and trust me, it’ll be special for all your guests, no matter their mothertongue! And your mom will get used to the fact that your FI is Canadian and she probably has to speak English to him – there are lots of students and international people in Stockholm and such these days, so I guess she’s probably a little used to it.
I hope this helps a little bit, or at least makes you feel a little better in that you’re not alone and other people have been through this too! And I LOVE the Swedish language and Sweden – I have heaps of Swedes at work, and I keep inviting myself over there for Midsummer and I just might make it one of these years! Also your Glögg, ginger biscuits, and cinnamon rolls are to die for – surely they can encourage FI’s Swedish along a bit?? ;D