I was shown a hang glider with mild sweep to the wings, and a few degrees of tip washout, that is, the angle of attack was slightly lower at the wingtips (maintained by tensioning the rigging, I think).
The idea of the washout was that the central part of the wing stalled while the wingtips (with lower AOA) were still flying.
And the idea of the sweep was that the wingtips being aft of the centre section, the centre of lift moved aft when that happened, allowing the nose to drop, automatically recovering from the stall.
So, sweep can be used to help stability, and I'm pretty sure that hang glider stayed well below the transonic region.
EDIT : Tip washout and mild sweep visible here around 0:40 and 1:20, and clearly low Mach number.