Working in that area in Japan. I think I can provide some answers.

Payment: CC are mostly used for BtoC but if you are a BtoB SaaS you want invoice and a local presence (ie no tax or currency shenanigans for your customer).

Hand on sales: Don't expect customers to sign up for a free plan and convert. Your conversion rate will be close to 0. Mostly scammers. Instead: Contact form, Cold call, go out to events, lots of drinking.

Regarding language: Many people do not speak English. I think that surprises some, but Japan is big and you can live forever happily only speaking Japanese. So if you don't support Japanese it's a complete no go.