Basically all but unconfirmed, you can count on Mario & Luigi Superstar Saga being real, but of course, without the official announcement of Nintendo themselves.
You may remember a few days ago some information was data mined from Nintendo’s servers, but now the games icon has finally been removed from the servers after the code was discovered. By the looks of it, we’ll most likely get a confirmation during E3. This year’s event runs from June 13-June 15. Stay tuned!