Since this is apparently the FM-Towns version, are the Japanese language files actually included?

The Japanese language FM-Towns version is not included. Just the English language version of the FM-Towns (VGA).
And the original PC EGA version (V2) of course.

You may be able to find Fujitsu FM-Towns CD Collection that includes Zak on the Internet Archive. But it is the FM-Towns bootable CD media. And would be up to you to figure out how to extract relevant data (ZAKKAN folder) from the CUE/BIN, and add it to SCUMMVM.
Note: The reason why the Japanese is not included is because you will also need FMT_FNT.ROM from an actual FM-TOWNS computer. Obviously old, but protected IP. Could find somewhere (mame bios/rom collections?). Or can recreate using a hard to get MS Mincho font and a utility (check SCUMMVM wiki).
You will also need the audio tracks if you want the music and ambient sounds. You would need to rip it from the cue/bin, named Track1.mp3 - Track23.mp3 I would think. There are files in GoG release, but there seems to be a discrepancy in the number of tracks between those and the FM-TOWNS disk image. Someone with more time could maybe compare them.

Thank you so much for your thorough reply!

Oh, it's too bad that the game is not available in Japanese. An original physical copy of it is now too expensive for me to consider buying. I would have loved to own a genuine copy, even if that meant just a digital version of it.