You don't need a power transformer (i.e. to convert 220v to 110v), you only need a cheap adapter made of plastic and metal just to make the plug fit in your wall/powerstrip sockets.
Like this:
https://www.amazon.com/dp/B002TZBW74/?tag=neogaf0e-20
All Switch AC adapters are multi-voltage and work anywhere in the world as long as you can plug them in.
Getting a local AC adapter is more convenient though. I'm in the Middle East and we have same plugs as Europe. I got my Switch from Japan (which uses same plugs as America), so I ordered a Nintendo AC adapter from Amazon.es to use on my Switch in portable mode (so that I don't need to deal with an additional plug adapter), and I'll keep the AC adapter that came with the console connected to the Dock all the time, since it will be out of the way and I won't be moving it doesn't matter. Best of both worlds.