On October 29, 1994, Super Double Dragon was put on the infamous German index by the BPjS. For more information about what this means and to see a list of games sharing the same fate, take a look here: BPjS/BPjM indexed games.
The developers (Technos Japan) originally planned to include in-game storyline sequences in the game (similar to the ones featured in the NES versions of Double Dragon II and III), but they were forced to rush the game out earlier and as a result, they were never added to the game.
Several years later, Muneki Ebinuma, the game's planner, revealed to a Japanese fansite, the proposed plot outline for the game. The storyline would be told through dialogue exchange between the Lee brothers and each of the bosses before and after battle, with animated face shots of each character displayed on-screen. The policewoman Marian, who is only mentioned in the manual and never appears in the finished product, would've made her appearance in Mission 4 chasing after the Shadow Warriors' truck (where she was supposed get captured by the bad guys) and later in Mission 7, after defeating Duke. The eventual plot twist would reveal that the Lee brothers and Duke, the boss of the Shadow Warriors, were childhood friends. After Duke's defeat, the player would've fought against Duke's shadow, similar to the doppelganger boss in Double Dragon II. An actual ending for the game was even planned where the Lee brothers talk to Duke's redeemed soul and later pay a visit to his grave.
In addition, more obstacles and areas were planned for each stage, along with automatic animation sequences (such as the player getting aboard the plane in Mission 2 by moving through a staircase). A complete soundtrack was composed by Kazunaka Yamane (the game's composer), but only half of the tracks were actually used in the game (a few of the unused tracks were featured in the Japanese version's sound test mode).
Seeing as how Tradewest couldn't wait for Technos Japan to finish Super Double Dragon, they released a slightly unfinished version out to the U.S. market. Meanwhile, Technos added the final touches to the game and released it in Japan some time later.
What are the big differences between the U.S. version and the Japanese version? I'll list as many as I can:
* Better vocal clips. Billy and Jimmy sound more realistic and younger.
* The music tracks play in a slightly different order.
* Slightly cleaner graphics.
* The Technos logo that appears at startup uses Mode7 in the Japanese version.
* Some unused music tracks from the U.S. version were finally used in the Japanese version.
* The game overall has a better balance of difficulty.