Are there other clauses which either side has failed to follow? What about other treaties (for the sake of scope, ideally only those that the US has been a part of)?
It does not say that Spanish and English will be co-equal, rather that Spanish speaking citizens shall enjoy rights to "liberty" and property. I'd imagine that having some official role for Spanish would be part of the former, but the US is not specifically obligated to put it on the same plane as English, then or now.