I fly to and within Japan pretty often and while there are always several fluent English speakers on the international flights, domestic flights are hit or miss (as you'd probably expect). I normally travel with my wife, who is a native, and on domestic flights they sometimes tell her she's just going to have to translate for me.
On the international flights on both ANA and JAL, they're not all equally fluent but they all understand at least the basic requests they might be asked, like for food or drink. But I have had some beyond just the purser who were both fluent, almost native sounding, and really talkative. I've never subscribed to the "Asian F/A's are all robots" trope. It's just that many of them only know enough English to do their jobs. But others know a surprising amount and are eager to use it.
That also reminds me of the most unexpected English experience I've had flying, which was at a Starbucks at Haneda Airport, in one of the domestic terminals. I was in line getting all ready to use my very limited Japanese to order, when I hear "Next! I can help whoever's next." No accent whatsoever. I look up and the cashier, who looked just like any other Japanese girl, is looking right at me. I said "You speak English!" and she replied "Of course! I work at Haneda Airport." Again, no accent, but I thought that was an interesting response. I ordered in English but didn't ask any more about hers. But after being surrounded by Japanese for a while, as well as the odd person who can just barely speak a little English, to have this person who was clearly Japanese but speaking English as if she grew up in Nebraska was really jarring, and she acted like everybody at the airport (in the domestic terminal) spoke just as well as she did. (They don't.) It made me wonder if they're all hiding something

I'm tired of being a wanna-be league bowler. I wanna be a league bowler!