Legal Question in Real Estate Law in California

The answer here pretty much depends upon what you mean by "legal." Lawyers (and judges) tend to reserve the concepts of "legal" and "illegal" to distinguish between conduct that can be punished as a crime and that which is permissible. Certainly, there is nothing punishable about a landlord and tenant making a rental agreement where the tenant has a limited command of the English language.

If language proficiency, or legal residency, were a prerequisite to renting property, we would have a lot of tourists and recent arrivals behind bars or out on the street.

In addition, not reading or not understanding a contract you sign is not a very good defense to the enforcement of the contract. A person who signs something is usually assumed to have read and understood it. This is not, however, an absolute rule. If the facts show that a party to the contract truly misunderstood it in some fundamental way, there is probably no "meeting of the minds" as required for a binding contract. However, merely making a bad decision or over-committing is not usually a ground to get out of a contract.

My hunch is that the rental agreement would be enforced, but I can't be very sure without more particulars and also direct discussion with the tenant here.

I agree with Bryan it isn't a problem nor a requirement that someone be "legal " to sign a lease. While the law requires certain contracts to be in the language of signer I do not believe that has been extended to real property yet.

The practical problem becomes when the tenant overstays their visa and the INS comes and takes them away effectively ending the lease and the ability of the landlord to collect rent.