When bringing the American version of an XBOX One S to Japan, how viable is it to buy games from a local Japanese store and play them on the American system? Are there region settings blocking that? Is there a difference in the programming that blocks it? I think this was an issue with systems from several years ago, but am not sure if that's the case here.
Based on this Reddit thread (and some other Google results) it sounds like the answer is no, the XBOX One is not region locked. (I assume this is also true of XBOX One S as they are considered the "same generation.") So you should be able to play games from anywhere on your machine.
However, take note that it sounds like DVD and Blu-Ray playback is still region-locked based on whichever region the console was manufactured for. I'm not going to link to the sources I found on that point, since they appear to be websites of questionable intent, and I'm not sure how Arqade views linking to such places.
Mind you, this only really matters if you're planning to use your XBOX as your primary movie machine (and you still buy movies on disc :P)
You should be fine. There's no region locking for games on the Xbox One, according to official Xbox support. However DVDs and BluRay discs are region locked, so you will have to be careful with that:
Each Xbox One console is manufactured for a specific Blu-ray and DVD region, following international standards. The Xbox One console can play Blu-ray discs and DVDs that are sold in the same region as the console. See the following tables for the countries in each disc region.
Some Blu-ray discs and DVDs are not region-encoded or are coded to all regions. Region-free Blu-ray discs and DVDs will play on any Xbox One console.
Note: Game discs for the Xbox One console are not coded to regions.