China DOS Union

I was localizing a software. The English in the graphical interface were all output using the puttextxy function, which doesn't support Chinese. Later, I came up with a way to write my own function that supports Chinese to replace it. I wrote the function.
But when adding it to the DOS program, I found some problems:
1. There's not enough space, no place to add. I wonder if I can add a section to hold my code.
2. Modifying the relocation table is extremely troublesome. The program is very large, with tens of thousands of table entries. I need to modify the relocation table to support the function I added.
I don't know if any experts have solved these two contents before?
Or is there any tool that can be used, like as easy as modifying PE.
I can handle adding functions in my own small program, but when it comes to the actual software, it's a headache. The software is too large.
Thank you very much and urgent help needed

Actually, it's quite simple! Compile an interrupt-resident program, hook it to an interrupt number that is unlikely to be called by ordinary software, and then in the places where needed in the software, call that interrupt number instead of a subroutine.