It definitely is just on the homeworld planet. There was a suggestion for decreasing % chance the further you got away from the homeworld, but didn't make it too far as it got too unwieldy.
It is still only an 80% chance on a homeworld (rather than 100/total number of races% that it usually is that is), so yes you will get other races as well.
Is it 80% or 90% chance when hiring from a homeworld? According to the rules, it's 90%:
“The race of each NPC is based on the planet where hiring and on the restrictions linked to the race. When hiring on a homeworld, there is 90% chance to get a NPC from the native race. In all other cases, the race will be randomly selected. Stormtroopers are human by default.