In Tokyo it is possible to get a job as a software developer without speaking Japanese. After living here a couple years someone decides they like life here, but want to run their own business. Maybe they want to run a SaaS or something that targets a global audience. For visa purposes, they need a Japanese company.
By this time they’ve maybe picked up up some basic Japanese but aren’t fluent. Becoming fluent in a language is quite challenging, especially if you’re working full time in an English language environment. Even without fluency, it’s possible to enjoy life here.
What makes you think that? Japanese is just a normal language. You have to study vocabulary and grammar and interact enough with native speakers to learn expressions. The writing system and the lack of cognates makes it harder for an English speaker than, say, Swedish, but there is no reason to believe that you can't ever reach full professional proficiency.
By this time they’ve maybe picked up up some basic Japanese but aren’t fluent. Becoming fluent in a language is quite challenging, especially if you’re working full time in an English language environment. Even without fluency, it’s possible to enjoy life here.