I do not have DQ5 yet (est. 25 min to go), but I feel the need to praise DQ4.
I do not read Japanese, but I had NO PROBLEM completing DQ4. If you use either the IPS patch or the language option in the cheat file,
everything you need to know is in Engrish! Only the opening bit with the frog/woman/rabbit is still in pure Japanese, because it was not
in the NES or PSX version. And it is not important. Just talk to the man in the house (your dad), and the story moves on. In Engrish.
If you wait for the US release, it will probably have much of the Puff Puff related dialog "cleaned-up".
I also recently completed Dragon Quest Monsters - Caravan Heart (J2E) on GBA/VBA. A full translation and a better game than Joker!
I do not expect to be able to play DQ5, yet, but ...
Edit: The code posted by keke_keke works. I got off the boat and leveled up a bit in the first cave. GameFAQs has a good walkthru for
Japanese PS2 version, so I guess this is probably playable, but I will wait for the US version. There are also several SNES resources.