I don't know anything about turtles, or what's safe for them.
But in our tortoise enclosures, some that do well and are safe are pothos, Boston fern, and prayer plant. Any kind of bromeliad is safe, as are African violets, but I don't know how well they would hold up to the heat.
I would think that any plants used in aquariums would be safe to have in your turtle's tank.
I've had Water Wisteria, Anubias, Amazon Sword, Duckweed, Water Lettuce etc.
Just be aware that turtles and tortoises are wrecking crews unto themselves! Plants that you plant in the underwater substrate are going to get bulldozed, dug up and pushed around! But the plants that float freely in or on top of the water will not be bothered. The turtle may eat them, or they might not.
How many African Side neck turtles in what size tank? I was never very successful incorporating plants in my tanks. I may have not had enough plants but after years of occasionally trying different plants grown locally in the inhabited ponds, I gave up. As Todd mentions, surviving multiple turtles in a tank is a hard row to hoe for any plant. They really seem to enjoy having effect on their world...lol