Great works! Thank you stef70。
I changed the code "en_US.UTF-8" in the rcS file to "zh_CN.UTF-8",
wishing this setting could solve the "????" in TXT file for Chinese character,
unfortunetly, failed, the KOBO Touch can't boot up.
So a hard factory reset was done.
It's strange, why kobo touch can render Chinese character correcty in EPUBs, but not in TXTs?