Wow, byteshaman! You're the one who should be praised, for such dedication! Most beginners would have abandonned by now. But I'm glad to be able to help you; you learn the most by trying, and I'm sorry to not have enough time to make this all a bit easier.
Well, back to business
Fonts (.TTF files) being in /home/tc/.fonts, edit your /opt/.filetool.lst file, and add this line inside:
home/tc/.fonts
(without the starting slash)
Loading xfonts-unifont is a good idea. However in this case (Japanese), I'm not sure it will be of much help. Just be sure to put the right fonts (with glyphs for Unicode codes corresponding to characters you want to display) inside /home/tc/.fonts/.
Looking at your screenshot, you
did achieve your goal, although you did so by changing the APPEND line at boot. To make it permanent, you indeed have to locate and configure your boot loader.
Assuming your bootloader is Extlinux, it should live inside a "boot" directory at the root of your boot partition. Thus, the right path to the config file
should be "/mnt/sda1/boot/extlinux/extlinux.conf"
Now, as both curaga and I said, you have to use applications that understand languages other than English. For the editor I suggest Geany: lightweight and featureful though not overwhelming
Concerning "/mnt/sda1/tce/onboot.lst", you're right: it's the list of extensions to load at boot. "mylocale.tcz" should be part of this file.
As for persistence, Assuming "home/tc" is not already in your /opt/.filetool.lst file, you'll have to add this line:
home/tc/.local
to get your symbolic links saved across reboots. This has nothing to do with the boot manager
Oh, and
don't,
ever!, put "usr" into /opt/.filetool.lst! You'll just ensure terribly long shutdown and boot times, if not a crash
Keep us informed. We'll help!