I'll try and look into both of these, but these might be a bit harder to narrow down exact information on.
I can help for some of the regional differences for the PS1 versions.
RE1:
In the japanese version:
- You have auto aim
- Enemies have less health than they do in the US/PAL versions
- Both Chris and Jill have more health than in US/PAL
- Each ink ribbon pickup contains 3 instead of 2 ribbons
- The intro is in black and white but otherwise uncensored, all the dog and Joseph gore is there, Chris smokes, etc.
- The Kenneth scene is completly uncensored
- You get a difficulty description in the character select screen. (Chris = Hard, Jill = Easy)
- All the files have a background image, while it's just a black background in the international versions
- Both the opening and ending use very cheesy vocal songs
Opening Song
Ending Song
RE DC:
- All versions are completly identical in regards to their difficulty, Original mode (Or Standard mode in some versions) is based on RE1's original japanese difficulty
- The japanese version now uses the same instrumental opening and credits music from the international versions
Other than that, all the same regional differences from before apply. (Well, apart from the uncut intro for the german and french versions...I still can't wrap my head around the fact that Germany of all places got one of the only uncensored versions for PS1.)
RE DC DualShock:
- The US version is slightly harder again (Enemies have more health and you have less...though the difference is not as extreme as it was in the original. Also, 2 ink ribbons instead of 3, again.)
Other than that, the same regional differences as before apply.
_______________________
RE2:
In the japanese version:
- Auto aim is always on, there's actually no way to disable it
- You have more health, enemies have less
- There are less enemies than in the international versions and they're placed differently.
- A lot of the ammo and health pickups are placed differently as well and a lot more obvious (No hidden dumpster ammo or ammo hidden behind the statue with the red jewel.)
- The game over screen is censored, it's just a black background with the "You Died" text
- The ranks go from S to D, while they go from A to E in the west
RE2 DualShock:
- In the japanese version there's an additional USA mode under "Arranged", which is basically the US difficulty mode.
______________________________________________
RE3:
In the japanese version, both Hard and Easy mode (Called Heavy and Light over there.) are easier, you have more health, enemies have less. That's it, the difference in difficulty is not as big as it was in the previous games.
And I'm not entirely sure, but I think they changed the ranks from S to D to A to E again.
Both the US and japanese versions were censored in various ways with later re-releases. For example, in the very first japanese release the first hunter you meet in the hospital would decapitate a zombie, in later revisions the zombie kept his head and there wasn't even any blood. You could still blow up zombie heads yourself though.
A special mention goes to the german version, there's no gore at all, the blood is grey and enemies just disappear after they're defeated. They also screwed up Mercenaries mode as you can only get a time bonus from killing zombie animals, and not from regular zombies, so it's basically impossible to get a good score.