In general all major airlines will have flight attendants on board that speak at least the following languages :
- The main language of the country they are flying FROM
- The main language of the country they are flying TO
That's not to say that every flight attendant on the flight will speak all of those languages, but there will be at least one that does. Many airlines have their flight attendants wear pins on their uniform that shows which languages they speak to make it easier to find someone that speaks a specific language.
The languages used for announcements varies depending on the airline and the route, but every flights I've ever been on to Japan has included announcements in both English and Japanese. For flights from South Korea to English speaking countries announcements are made in both Korean and English (Based on flying both Asiana and Korean Air) - I don't know if flights to Japan would include Japanese or not, but I strongly suspect they would.