Parts of this interview have already been translated, actually.
One part is about Bowser Jr. almost being cut, and another is about Greninja's development... again... Sakurai really likes Greninja, doesn't he? I'll edit them into this post. One sec.
EDIT: Here you go, straight from NeoGAF.
This is from pages 3-4. ND is Nintendo Dream, MS is Sakurai. Here's the link:
http://m.neogaf.com/showpost.php?p=145444294&postcount=63
ND: When not you are not developing and there is a new title or character released, you are not thinking about "How about that one in the next Smash Bros?", right?
MS: Absolutely not! I'm always thinking that doing Smash Bros. again will be impossible. Impossible, impossible...is what I always think, but I ended up making it again (laughs). But once I decide to do it, I'm very fast about creating moves and such. For example
Greninja, even before his name was decided I received several illustrations.
I took them home in the evening and around midnight I had already done all his actions, normal moves, special moves and pose-pictures and sent them around asking "What do you think?".
ND: That's incredible speed! By the way, when deciding on which characters to use, are you looking into unreleased new games?
MS: At the very beginning I did that.
This time our project-proposal is dated May 2012, at that time all characters were decided already. Then as production moves on we will say "We won't put that chracter in" and cut out low-priority-characters.
ND: That means in your project-proposal there are more characters than ended up in the game?
MS: Yes, but I won't tell which of course.
ND: (laughs). We were wondering all the time "Which characters will be in", and then the "long-time heroes" are announced just like that. We were wondering if that was the right way to announce them.
MS: After all we were planning on including so many characters, in the end this pace of announcing them was enough. Each and every character has fans, we wanted to drop as few as possible. About the order of which character has priority, the characters that don't have a new title coming up have an overwhelming disadvantage...even characters that we ended up including could have been left out if development had progressed differently. But even if 1 former character is left out, for the fans this is a huge thing. On our side, we are re-creating characters from the previous title, and keep on adding more, so the word "reduce" is not appropriate. There are cases where we simply couldn't make it in the end, but on the whole we did a good job I think and the people at Bandai Namco Games did a great job. We had discussions on giving up on something many many times,
Bowser Jr. was on the brink of being cut but the staff said "We'll do our best!" and we made it.
ND: Were there Smash Bros. or Nintendo-fans among the staff of Bandai Namco Games?
SM: Of course. People who love Nintendo, people who love the characters, people who aren't a fan of something specific but love games as a whole...it depends on the individual.
ND: Was there something you didn't ask for but got created because one of your staff was obsessed with it?
MS: There are several cases of this. For example, Sheik's movement is completely different to Brawl, someone made a proposal to me for those motions.
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The translator also left a nice little comment I'll include with the post...
"So once you hear about a new Smash Bros. being in development you are not asking for which characters to be included, but which characters shouldn't be cut."