Tech in Japan: how do I do it?

It's a bit similar to Texas.

There are a small and slowly-growing number of firms in Japan at which English is the workplace language. You might want to try researching them first -- not sure if any of them are in the embedded/firmware space, but it's worth a look.

Other than that, if you're serious about living and working in Japan then you should have at least a basic conversational ability in order to get by in everyday life. And if you work at a company whose workplace language is Japanese, then you'll need to have that ability -- probably ahead of time, and probably to at least the N2 level of the Japanese Language Proficiency Test.