may I also add that it may also be MTE compressed, which would explain why you can only "decode" small parts of sentences. *looks around for a table*
Ah yes here we go. Lookie
here. This is a partial table of Phantasy Star IV. You see the byte values at the top? That's MTE. See those values at the bottom? That's also MTE. Note that this table is incomplete. What you would need to do is do a bit of hacking to find out what the rest of the byte values are.