Sorry I can't help you directly with your problem because you haven't presented a reproducible test case that I could look into. That being said, let me make a few observations.
tr uses harfbuzz for text shaping and that only supports unicode text as input so if you have fonts with other charset encodings and you try to pass text in anything other than unicode that won't work.
the tr and ui are being reimplemented in a combination of Terra and Lua (see my previous announcement) - when that will be finished, the current pure Lua implementations will be dropped, so you might wanna wait for that? dunno, it's up to you, but FYI.
why don't you want to modify the code? this code is written by someone who knows nothing about chinese and who just expects harfbuzz and freetype (and of course the fonts) to do the right thing, so you'd better be willing to roll up your sleeves and dig into the code if it doesn't work right (I'm thinking maybe it's time to make a dedicated post just on this topic, just to adjust some expectations)