From the little I did on PSP, most games I've fiddled with just used the system font and some basic non-fancy encoding for text, making text searching and editing rather easy (to the point where I a few years ago could pretty much whip together cheap-o menu patches for stuff I played around with using just a quick bit of simple search and replace).
However, aside from attempts at fiddling about with other stuff; most crashes (and other odd misbehavior) I encountered were the results of text changes where the length of the final output was an odd number istead of an even one.
So if you say...
The Japanese wrote something that in data would appear as
C4B3 C4B2 C3A1 C311
That'd be a total of 4 double-byte characters, which most likely gives you 8 letters in your western single-byte alphabet.
Let's pretend you only need 7 of those for your English translation.
You wanna write
"Bonkers" or maybe "Dorks" ... I don't care ... doesn't matter.
If you experiment with the memory values in the range you found the Japanese font set in, you'll likely find a double-byte set for A-Z and a-z (to go along with their single byte equivalents that you're probably currently using). So the way I fixed that stuff (in a cheap and easy way) was to write the first letter in double-byte (when needed) and the rest of the word in single-byte. Making Bonkers (7) and Dorks (5) technically be even in length (8+6) and not odd. Of course, it could just be that you need to write everything in double-bytes... (and the single byte A-Z/a-z just aint supported by the game - unless you go more indepth and hack in support for that...)
At least that's one of the reoccuring issues I've had with simpler PSP games and it's a fairly easy work around... assuming that's the problem you're experiencing now.