At this point, we have very little information to go on regarding Pokemon Switch, but a recent job listing has shed some light on the development of the game. It heavily hints that The Pokemon Company is using either Unity or Unreal Engine 4 to create the eighth generation of mainline Pokemon games.
In the job listing for a 3DCG Designer found here, it is noted that the designer must know how to work with Unity or Unreal Engine 4. While this could simply be a requirement so that the person will have the necessary experience to work on a different proprietary engine, there are a few interesting details hidden here worth noting.
First and foremost, there's the credibility of this job listing for Creatures studio. Creatures is a Japanese studio that just earlier this year in June opened a new office that is specializing in Pokemon CG. That is fairly recent, so it stands reason to believe that the studio must be assisting with development on the upcoming Pokemon Switch game.
This lends context to the picture above that is attached to the job listing, showing Pikachu looking better than ever. It is possible that the picture can give us an idea of how Pokemon will look on Switch. This could also be a prerendered cutscene given the purpose of Creatures studio, so it could be not representative of the rest of the game.
Also, it wouldn't be far-fetched for Pokemon Switch to use Unreal Engine 4, as there is another marquee Nintendo title that is confirmed to use it. It was revealed (via IGN) earlier this year at E3 2017 that Nintendo's first party studio is using Unreal Engine 4 to develop the new Yoshi game for Nintendo Switch that is expected to release in 2018. As of right now, there is no release date confirmed for Pokemon Switch.