I’m a bit of a travel hacker, and I recently came up this trick for finding cheap flights out of Japan.
If you use Skyscanner or Kayak (and similar aggregators) search with the language settings in Japanese, you may well find flights that are a lot cheaper than if you search in English. If one of the partner airlines only has a full booking process in Japanese then these flights won’t appear in the English versions of Skyscanner etc. So you often miss out on many of the cheap flights.
Using this trick, I was able to get a return to Europe with HIS Japan a full ¥50,000 cheaper than on the English version of Skyscanner. HIS Japan is just a travel agent and the airline booking was actually with BA, so once I had purchased the ticket I was able to get the BA reference (by emailing HIS Japan in English!) and then register on the BA system in English. So now I can checkin using the nice BA mobile app, order a special meal etc - no need to fiddle with some legacy Japanese website.
The only downside - HIS keeps sending me promotional offer emails in Japanese, and apparently there’s no way to opt out!