Well, most games read their text byte-by-byte when displaying it to the screen. So you'll have a bit of code that reads a byte from ROM, then does something with it (loads some font tile or other, updates a tilemap somewhere, etc.) You want to intercept the game as it streams through its text, and test each byte of text as it gets read to see whether it falls within your DTE range. If it DOES, that's when you start flag-flipping and feeding other values not in your DTE range (iow, regular text bytes) back to the game.
So... sort of like this:
1) Game loads a byte in its text stream.
2) Check to see if the byte is within your DTE range (90-B0, using your example.) If it's not, return to the routine and do nothing.
3) If it IS within your range, flip a flag to tell the game you're in DTE Mode, load the first value in the corresponding pair, decrement the pointer to the current spot in the text stream (frequently x, y, or some register being accessed indirectly) then return to the routine with your new value, the first in the pair, and let it do whatever it was going to do with the value in the first place.
4) Next time the routine comes around and the game loads a byte, you'll still be loading the same byte, so it will be in your DTE range. Your code should notice that you've toggled your flag, and flip it back. Then you just load the second value in the pair and continue as normal.