>>96219373 (OP)
Set a detail in your world that either it is in a place where everyone is following the currently-strongest nation's sphere of influence ("Common" is just the official language of the empire, so everyone is forced to standardize anyway) or that your not!Atlantis/Hyperborea/SuperRome that eventually fell managed to conquer the entire world and create a standard of measurements before falling, so the current global standards are based on that (with local ones still existing in holdouts).
In either case, there could be a problem caused by the difference in measurements that leads to an incident for the party to solve.
>1 [Standard inch] is essentially 2.07 [Local inches]
>everyone just makes it 1:2 for ease of use
>something requires 100 Standard
>novice messed up converting between Standard and Local
>does 200 Local when he should have done 207
>squabbles ensue, party has to step in
Something smallscale that can establish it as a thing NPCs have to worry about, but it would be presumed to be corrected by the time it gets to PCs from then on, like how we don't tend to question payment processors auto-converting between foreign currencies.