Can something be “simulated” without mathematical language to describe the relationships and rules of the aspects of the simulation? Can a complex phenomenon– a complex system–be simulated without formulae?

Sometimes, but with one major caveat: there must at least be data. Without data, the simulation would be purely fictional without undergirding, and therefore not really be a simulation of some thing at all, but rather a new thing in itself.

So what if the data is incomplete? Does that make a simulation (at least in the traditional sense) impossible?

Maybe.

Not good.