Languages are the best.
In my world:
A ‘detour’ is most likely to be a (probably voluntary, although of course unplanned detours are a thing) change to a route (most likely to visit a relative, or take in a museum, or go to a beach or something), on your way to somewhere else.
A ‘diversion’ is either an official change of route for traffic because a road’s closed, or something created to distract attention from something else.