Step 1: ensure you have lots of time and patients
Step 2: download a hex editor and a file extractor (likely quickBMS)
Step 3: learn how to use quickBMS and write a script to extract all necessary files from proprietary archival formats used for your games files
Step 4: hex editor! Open files one by one and look for stuff that might be text or compressed text files
Step 5: if you determine they are compressed write another script to extract those...
Step 6: once you have found, extracted, found a way to edit, and edited all the text (mind you some things are likely parts of an image) you then need use your quickBMS script to repackage all of your files back into your proprietary archive format and test.
In short, it's not easy. You can do it but prepared to spend weeks, months, possibly even years doing it especially if you're doing it on your own.