I fly UA to/from Japan all the time and most of the Japanese speaking FAs are clearly native (note: I live in Japan and have done stuff like simultaneous interpreting so yeah, I can tell). Sometimes there are nonnative speakers but I can only think of one flight recently where the person obviously wasn't native (might have been AA though). Actually, sometimes the UA Japanese FAs aren't good at English, so I have to communicate with them in Japanese. NH and JL flight attendants are generally terrible at English and they basically only employ Japanese (last flight there was a Korean, although I highly suspect her family had lived in Japan for generations).
There are two GAs for UA in Chicago that I usually see at the NRT flight (and the NH flight to HND) that are obviously native speakers as well.
It is generally hard to find and keep employees who are good at two languages for many industries, not even considering the job and pay of an FA.