That's a lot of questions.
1. the final part, Part 3 isn't out yet, and doesn't have a release date yet.
2. The game is mostly translated. The full story up to chapter 2 and the Collab scenario are translated. what is slowly being finished is less important stuff like each monster's battle cries, or reactions to certain gifts.
3. After completeing upto the end of part 2. you can continue playing around in the world, but you also have several new things to spend time on. The collab scenario requires you finish part 2. there is a endgame superdungeon to play with (it can get crazy, with bosses with health in the 100 millions), You can also go into NG+ if you want. where you can choose the other route in the beginning and recruit some exclusive monsters.
4. Black Alice is an optional boss in part 2. she has 3 phases that you have to fight back to back, so it's very much recommended to save her for later.
5. I'm assuming by old saves you mean transfering from part 1 to part 2? yes, they transfer, there's no need to NG+. Part 2 has everything part 1 has, except for the images from part 1.