The Spanish Alumbrar means “to light up” in English — and, indeed, it is literally the same as to illuminate.
The Latin m-n sound almost always became a m-b-r as Latin turned into Spanish. Compare hominem with hombre, for example.
We see the same pattern here. Both alumbrar and illuminate come from the Latin luminare, meaning the same, “to light up” — from which we also get the English luminary.
Thus, the l-m-n in the original corresponds to the ll-m-n in the English illuminate and the l-m-b-r in the Spanish alumbrar.