Sounds like a fun trip. What area of the country will you be going to?
Switzerland is a very cosmopolitan country, German, French and Italian are the national languages, obviously not spoken by every single person though. Some areas are more French, other more German, in the bigger cities English is widely used and understood. Younger people have a good grasp of English as it's taught in schools and it's compulsory to also learn one of the other national languages. And with Europe having no borders, you'll find a lot of different nationalities live there too.
Switzerland is NOT a cheap place to go to or live, but they do have the highest wealth per person of any country in the world and have the second highest world life expectancy too. They're also the largest consumers of chocolate in the world, lol! Swiss Chocolate is wonderful!
Food is good but only you know if your boys are picky or will try new things. As to taking an extra bag or taking food with you, only take dried food. I don't know if you've flown internationally recently but the airports have really clamped down and security is high. Lots of things are no longer allowed.
Yarn? Lang is Swiss
and most of the yarn stores in Switzerland stock it, some almost to the exclusion of other brands! It can be expensive too, just because nothing is cheap there, not because there's a weird wool tax or something!