If you aren’t already here, you’ll be at a disadvantage to any other candidates who are already here. You’ll also miss out on job opportunities in the first place, as many interesting development positions recruit by word of mouth. So if you want to live/work here, the best thing to do is actually come here. New Zealand has a working holiday visa agreement with Japan, so if you’re under 30, that’s one easy way of getting here.
As for language, it really depends on the kind of company / position. Obviously if the company is entirely staffed by Japanese, you’ll need to speak Japanese to get a job there. On the other hand, if the company already has an international developer team, you won’t necessarily need any Japanese skills. In my case, I didn’t have any Japanese ability when I came here, but as the development team I worked for communicated primarily in English that wasn’t an issue.
Getting hired here comes down to the combination of your language and technical skills. If you’re a fluent Japanese speaker you don’t necessarily have to be a great developer - you’re ability to communicate between Japanese and international developers should be enough to get a job. On the other hand, if you don’t have any Japanese skills, you’ll need to be a pretty good developer to get a job here.
Another option to come here is the JET Programme where you’ll act as an assistant at Japanese schools. I know several developers who started out as a JET and then became developers. Because you have a fair amount of free time, and you’re often placed in areas without much of an international presence, it can be a great way to ramp up your Japanese skills.