I am fluent in English (native level) and work entirely in Japanese at a Japanese company. I however have a job, live in Nagoya and am not ethnically Japanese.

I was also curious about the requirements saying that the applicant needs to be a developer, have large-scale data experience and be good with customers.

So you (the OP) need a native level english speaking Japanese person with design and programming abilities who can also manage large amounts of data and can also sell the product.

I wish I knew where that person lived!

But I agree with your overall point - native level in either language is not necessary but we are looking for someone who can deal with Japanese customers, code and communicate well with the rest of us. I'll update the docs.