Region Free on disc-based titles and digital titles, Region Locked on DLC
All Switch game cartridges are region-free, and will work on any Switch. However it’s worth noting that DLC for both physical and digital games is not, and there is no way around this at the present time.
To get digital titles from out-of-region, you’ll need to make an account for the relevant region, and have a way to buy that region’s point cards – the easiest way to do this is from an online store that will ship American/Japanese point cards internationally.
However, once downloaded onto the system, every account whose primary console is the same one will be freely able to use that game – i.e. you can use the other-region accounts simply for purchasing, and your main account to play the title.