This will depend upon many things. First whether the lua you have is compressed or turned into bytecode type setups (puzzle quest on the DS being an example of this).
Many a game using lua has its strings in the same files, however the reason we often use separate text files is because you don't want to have to compile a whole game if you made a typo or want to change phrasing. To that that end it is entirely possible to have to translate a lua using game as you would in a more conventional ROM hacking scenario (pointers, encodings and such). Speaking of encodings I have in the past seen games (Zombie Daisuki on the DS) where you had mixed and matched strings of different encodings and you would have to play to that (or edit the whole game).
So yeah step one is open up the lua files in a text editor and see what you can do, and if the text is in them. After that it is conventional ROM hacking territory, give or take said lua files being compressed with a known compression with tools for it.