I've not translated everything, but most of the important bits/things that interested me.
-The producer and director are the same as the last three games. The producer was the director on the original.
-About six months after Bowser's Inside Story the team was throwing out ideas, one of them became the basis for the dream system.
-They considered going with Bowser again for the Dream System, but decided to limit themselves to Mario and Luigi as playable this time so the series didn't loose its roots.
-The idea to have many Luigis battling and moving in the overworld was a separate one that came before the dream system, but they wanted a system that used the second screen.
-The team hadn't considered this game would come out in the 30th anniversary of the character and be part of a 'Year of Luigi'
-To get the DreamTeam Alpha up and running too 1.5 years for Alpha Dream. Game has been in development for over 3 years, since the last one.
-They had so much stuff this time they even went into a Gamma Phase, which Iwata notes as unusual as games typically only have Alpha and Beta. The reason was the switch from 2D to 3D backgrounds and the length of time they spent on the game.
-They chose sprites because it's part of the Alpha Dream heritage...but also because they had few 3D production resources
-The 3D Mega Luigi model was done by a 3rd party after they realised there was a limit to doing that in 2D.
-A challenge was shifting from drawing sprites for 8-way movement on DS to 16 way movement on 3DS.
-They couldn't even just flip the sprites for the other direction due to the 'L' on Luigi's hat.
-They banned themselves from flipping the sprites for Mario too, because of Mario's right arm going up when he jumps.
-They had 5-6 staff working on the character animations for all the characters in the game.
-The requests and suggestions by Nintendo on Alpha Dream were not always accepted and often required meetings, or discussions with the SPD producers. Sometimes both parties got annoyed with each other.
-They talk about including an easy mode. For example, when active, a hint block will appear when you redo a battle after dying, or a Slow Mode will activate if you continually fail a brother attack so you can time your move more easily.
-The Mario & Luigi RPG series is also quite popular with women.
-Many of the characters from older games appear, too.
-A Hard Mode unlocks for experienced players after clearing the game. Even the producer has trouble with it, it's so difficult。He even died during the tutorial.
-The put a lot of extra stuff into Dream Team after hearing fans say they wanted to play more of M&L RPG 3. There's special requirements for battles, like 'no damage KO' which will earn you points, once you have enough points you will earn a reward.
-Worked hard on the tempo of the game, especially the opening. Redid it multiple times to get it 'right'.
-The producer says long term fans shouldn't worry - the series is still the same, despite the switch to 3D worlds for Dream Team