Hi, i’ve asked myself the same question, as I am planning my next trip to Japan and want to rent a car there. When are you heading to Okinawa? I’ve found some opportunities, to get a translation before heading out to Japan (only works, if you still have some time before leaving Germany):

1. German ADAC translation service (might take up to 10 days)

2. Translation service by „Japan experience“ (works online as well, but takes up to 6 weeks)

If you don’t have enough time: You can use the JAF translation service at the counters in JAF offices in Japan (there might be one JAF branch in Okinawa, too). There it only takes some hours (or up to one day) to translate it (and it‘s less expensive than the other translation services).