I was going to try to answer these questions one at a time, but I think it's easier and less repetitive to just cover it all at once.
Playing games
Xbox One is region free. Any games you own digitally or physically will work just fine, regardless of your console's region. The only exception are some Chinese titles, but I won't get into that here since it doesn't apply to you.
And yes, that applies to backwards compatible games as well.
Buying games
This is the main thing that is affected by your console's region. If your console is set to US then you will be using the US marketplace, meaning you can only buy what is available to US customers and you are paying in USD. If your region is set to Japan then you will be using to the Japanese marketplace and will be paying in JPY.
There are a few things worth noting here:
just like on PSN, your account has a wallet of all the different currencies and yes you can buy gift cards to give yourself credit, and use that credit to buy games. You will not be able to use credit to buy 360 games however, that can only be done on an actual 360 console.
if you use the Japanese region, if you want to buy CERO-Z rated games (ESRB M equivalent) you will need to make the purchase with a Japanese credit card; a credit of JPY on your account cannot be used for these.
if you use the Japanese region, you will not be able to buy any backwards compatible 360 games. Although the Xbox One marketplace is unlocked, the 360 marketplace is not, meaning your US account can only buy games from the US 360 marketplace, and buying backwards compatible games still goes through the 360 marketplace.
even though there is no more region locking, there are still region specific versions of games released, usually for censorship reasons. Japan is one of these countries, so if you want to play with your US friends you will want to make sure you buy from the US region as these are considered separate titles, and you will not be able to play online with people using the other version. A popular example that would apply here would be GTA 5.
Game pass
Worth nothing I suppose because I was just talking about the marketplace. Like the marketplace, the games available on game pass are also dependent on your region. Game pass just launched in Japan a couple of weeks ago.
Language
For the system itself (home/guide) you can set the language to English, and this is a separate setting from the region. It is possible to have your region set to Japan and language set to English.
For games, I'm not sure. This is the first time I've heard of any games automatically changing based off your system settings, and I don't think that's actually true. But if it is, I would assume it would change automatically based off your console's language rather than your console's region. If you are interested in any Japanese backwards compatible titles, I would do a quick google search first to see which languages are supported, because some games do have English and some are Japanese only; it's really just game dependent.
tl;dr you should be totally fine with your console's region set to US.