Short answer. Yes, within a narrow limit. Article 786 lets you leave property or money in general to specified classes or causes and entrust a third person to choose the actual institutions and to distribute it. What you cannot do is hand someone a blank power to decide who inherits your estate at large.
What the law says
The testator may entrust to a third person the distribution of specific property or sums of money that he may leave in general to specified classes or causes, and also the designation of the persons, institutions or establishments to which such property or sums are to be given or applied.
Civil Code, Article 786 — A Narrow Exception for Charitable Classes. Read the full provision →
What Article 786 permits
The article draws a careful line: the testator may entrust to a third person the distribution of specific property or sums of money that he may leave in general to specified classes or causes, and also the designation of the persons, institutions or establishments to which such property or sums are to be given or applied. You supply the substance — a defined amount, and a class such as children's hospitals in your city or scholarships for indigent students. The third person supplies only the detail: which hospitals, which students, and how the fixed sum is split among them.
Why this is an exception, not the rule
The general principle is that making a will is a strictly personal act: the testator, and no one else, decides who takes his estate. Article 786 is a deliberate exception to that, and it is kept narrow for the same reason the rule exists. You may delegate the picking of beneficiaries only inside a class you yourself have specified, and only as to property you have already set aside for the purpose. The choice of the class, and the decision to give to charity at all, must be yours in the will. What is delegated is execution, not the disposition itself.
Where the limit bites
The clause that does the work is specified classes or causes. A gift to whatever charities your executor thinks fit, with no class named, hands over the disposition itself and falls outside what the article allows. The same is true of leaving it to a third person to decide whether your estate goes to charity or to your relatives — that is the very choice the law reserves to the testator. Draw the class tightly enough that a reader of the will can tell what you intended, and the delegation of detail underneath it is valid; leave it open, and the provision is at risk.
How to word it so it holds
If you want a trusted person to allocate a charitable gift, say so precisely in the will itself. Fix the amount or the specific property. Name the class or cause with enough definition that it could be identified without the chooser — the geography, the kind of institution, the purpose. Then name the person entrusted and, sensibly, a substitute in case that person cannot act. This is work for the drafting of the will, not something to leave to a side letter, because a wish expressed outside the will carries no testamentary force. A clause built this way gives your chooser room to act while keeping the disposition yours.