[Update] Nintendo has provided an official comment to IGN on the matter, telling the outlet:
There seems to be some buzz online today about the launch date of Super Smash Bros. We would like to clarify that the speculation about a Spring 2014 launch date for Super Smash Bros. on Wii U was the result of a misquotation in an interview. The original article has since been rectified and we have no further announcements to make at this time.
So, as of right now, all we can say with certainty is that the new Smash Bros. game will launch sometime next year.
[Original] The latest entry in the Super Smash Bros. series will be launching sometime during the spring of next year. That is, if a slip up from a Nintendo exec is to be believed.
German Nintendo General Manager, Bernd Fakesch, shared the following statement with Neckar-Chronik (via NeoGAF): "For next spring two more big license games are also announced for our TV console with Mario Kart 8 and Super Smash Bros."
While he may have just misspoke, there's a reasonable possibility that the game will be out in the earlier half of 2014. At least, you'd think Nintendo would be pushing to get the game out as soon as possible considering the current state of Wii U.