Short answer. No. You cannot draft a disposition so that another person decides whether or not it operates. Whether a gift in your will takes effect must be settled by you, not delegated to your executor or anyone else. The essence of a testamentary act is personal and cannot be handed off.
What the law says
The testator may not make a testamentary disposition in such manner that another person has to determine whether or not it is to be operative.
Civil Code, Article 787 — Validity Cannot Be Left to Another. Read the full provision →
The decision has to be yours
Article 787 states the rule directly: the testator may not make a testamentary disposition in such manner that another person has to determine whether or not it is to be operative. Making a will is a strictly personal act. The core choices, what you give, to whom, and whether the gift stands at all, express your own testamentary will, and the law will not let you pass that judgement to a third party. A clause that says "my executor shall decide if my nephew receives the land" hands the yes-or-no to someone else, and that is exactly what the article forbids.
Why the law insists on this
A will is meant to be the testator's own disposition of his property, formed while he is alive and able to weigh it, and preserved because it cannot be confirmed with him afterwards. If another person could decide whether a gift operates, the effective disposition would be that person's, not the testator's, and made after the testator can no longer speak to it. That defeats the whole point of requiring a will to be personal and in proper form. So the prohibition is not a technicality; it protects the disposition from being quietly rewritten by whoever holds the power to switch it on or off.
What you can leave to others
The bar is on delegating the substance, not the mechanics. An executor may be trusted with the ministerial work of carrying the will out, gathering the estate, paying what is due and delivering each gift as directed. What he may not be given is the discretion that decides whether a disposition exists in the first place. The line is between executing choices you have already made and making those choices for you. Administrative judgement in the course of settling the estate is fine; a veto or a switch over whether a bequest takes effect is not.
Use a condition, not a delegate
If you want a gift to depend on something, tie it to an event rather than to a person's say-so. You may lawfully make a bequest conditional, so that it takes effect if a stated circumstance comes to pass, because then it is the fact that decides, objectively, not another individual's will. Draft the condition clearly enough that whether it has happened can be answered from the facts alone. That achieves the flexibility people reach for when they think of naming a decider, without running into the prohibition, and a lawyer drafting the will can frame it so it holds up.