It really depends on the game.
As a start, I'd recommend running the game in PPSSPP on Windows (or using WINE). Then get to a place that shows some text, and open the memory viewer. From there, you can right click and dump the RAM to a file.
Now you'll want to use a hex editor and check that file for the text. It could be a lot of different ways:
* Encoded as shift-jis
* Encoded as utf-8 or utf-16
* Encoded using a custom encoding
Once you find the text in RAM, the next part is made much easier as long as you're using PPSSPP v1.12.x or later. Go back to the memory viewer, and go to the offset of RAM (0x08800000) plus the offset where you found the text in the RAM dump.
From here, there's a dropdown that allows you to see allocation, sub allocation, write, and texture info. Change it to write. Next, look at the label in the status bar when you have the text selected.
It may say something indicating the filename this came from, or where this was copied from. If it doesn't, you may need to use a memory breakpoint, and repeat the scene where you see this text. Ultimately, your goal will be to trace this back to where it comes from.
You'll probably end up seeing functions that you need to understand (you might use Ghidra or just read the Assembly for this.) In the easiest case, it will just be read directly from a file, but it might also be decompressed or converted from data in the file.
Games can store text all sorts of ways, and some of it will just be stored as raw images. The GE debugger in PPSSPP can also help you identify this.