I have the PS2 version and though I don't currently have a PS console other than my PSP, you need either a Japanese PS2 or a U.S. PS2 with Swap Magic (CD and slide card) to make the game work. So yeah, you'd need to mod your (U.S.) PS2 to get the game to work.
As previously stated in this thread, both the PSP and Nintendo DS are region-free for video games.
-Is there any version available in English? My guess is no, so follow-up questions:
NO EVA GAME has been released in English EXCEPT for a fan-translation or FAQ here and there.
-Are there any guides for English speakers that help with translations?
-What's the worst thing that can happen to Rei? Can you make her angry or very depressed? It would be fun just as a side thing to do whatever is possible to hurt Rei's feelings and then see something tragic happen to her.
AFAIK, the worst that can happen is Rei getting her canon EoE end of her dying as Lilith. In actual gameplay, you can raise Rei to be a delinquent (ignores the orders you give her, etc) and she runs away from Sani then returns to him in the rain.
I think Rei being a delinquent is also when she acts somewhat cocky (Asuka-ish I guess you could say) but I'm not sure.
I haven't gotten that sequence, when I played through the game I was focusing on making Asuka and Rei cheerful. (Rei sounds so funny talking all energetic, it reminds me of Mana heh.)
The games are fun, if Raising Games are your thing. In some ways I prefer the Asuka and Ayanami game to the Shinji Ikari game because of the use of anime-like stills, although the Shinji Ikari Raising Game is somewhat more interesting in terms of the information provided during the game. (Misato reveals to Shinji that her father loved cars, and how Misato's a bit of a car buff herself, for one thing.)