There is a solid emulation base and comunity for the emulation of japanese computers, and although nearly all emulators came from Japan many of them have english menus available.
Japanese emulation scene is very different from western one, most emus are closed source with barely any hint about the identity of the creators, the GUIs are usually custom made and strange, and so on. Also many emus are old and not manteined anymore.
There's whats available for the computers you asked about:
- PC98: There are three main emulators, Anex86, T98 Next and Neko Project. Anex86 is probably the best known and the easiest to use, but to me is also the worst of the lot (it's not bad, but the others are better); T98 Next is the fastest and has good compatibility but its GUI is an example on how NOT to do things, unintuitive and convoluted like few I've seen; and Neko Project is the most compatible, the only one that's still updated and the only one who has a typical windows menus, but it's the slowest and also it has its quircks.
- FM Towns: Basically there's UNZ, it's easy to use but it has bugs in some games and it's not been updated in nearly eight years; lately MESS has been doing progress in FM Towns emulation, although I'm not sure in wich state it is now, it's not a platform I care for.
-X68000: My favorite japanese computer, there are basically two emus available: XM6 type G is the most compatible one, but the image always rendered on a forced 3:2 aspect ratio instead of the correct 4:3 ratio,apart from that it's an excellent emulator; winx68k high speed is a bit faster but it's not as compatible or accurate and has some bugs, also it's an old emulator (the codebase is from the win98 era) and in windowed mode the size of screen is too small for today's monitors and not adjustable.
There are some more emulators and some more machines, but this covers the basics.