On top of learning martial arts from TV and movies, Splinter probably picked up English from there as well, as he has a Chinese accent, similar to his actor. On top of that, the turtles being surprised that Splinter can understand Scumbug mirrors the fact that Jackie Chan knows other languages besides his native Cantonese and English.
Splinters kiss with Scumbag is pretty damn gross on many levels, but here it does make sense. He is a mutated feral sewer rat, with none of Yoshi's human sensibilities either ingrained as a man or learned as a pet.
It's not hard to imagine how disastrously Superfly's machine would reallyscrew the world up if all went to plan. For just some of the many problems it would cause:
Every animal and insect on the planet (insects alone, are at an estimated 10 quintillion, about 125 million times the number of humans) becoming a human-sized mutant would lead to an Overpopulation Crisis of unfathomable proportions.
Another would be the issue of food as both prey and predator would be given sapience. That's a whole new can of worms that gets worse the more one thinks about it.
April should really count her lucky stars that she wore a helmet during the Ninja Star scene. If she didn't, things would have been very grim.
The ending seems to suggest Superfly reverted back to a normal fly. Keep in mind that houseflies have a very short lifespan, 28 days for certain circumstances. Splinter is lucky to have come into contact with the mutagen; otherwise, he would have died years ago.
After Superfly is defeated, the creatures he fused with are released from his bio-mass. While the horses and such are able to get away on their own, the sea creatures such as the octopus and whale end up beached in the middle of Manhattan with likely little to no prospect of making it back to the ocean alive.
