I made a test and I have to confirm - it can't be done this way.
I suggest to install sparky on a virtual machine or a additionl partition with your locales.
Then install fonts, fcitx, live-installer, sparky-backup-sys and add grub-efi files as the Sparky-backup-sys info says to your home directory.
Then build a new iso from the installed system.
If any problems, just let me know.