This is, of course, part of the broader tendency of legal Spanish and Spanish in general to use pronouns and synonyms to avoid repetition. The use of synonyms for the defined terms and party names being another common feature of Spanish contracts.
So, what’s the implication for translating contracts? Simply, don’t be worried if you find yourself adding in the party names more often in your English translation than occurs in the original. Replacing the Spanish pronoun with the party’s name is actually a good practice and will make the translation appear more like an equivalent English contract. As is making the party names and defined terms uniform throughout.
And it’ll certainly make it more readable and clearer. Is that something you want? I’d argue that it nearly always is.
However, regardless of any argument you might make about faithfully maintaining any trace of ambiguity in the original, you can often feel you’re removing ambiguity when it’s really a case of ambiguity only seen by you (which might come from reading the original too closely — as is bound to occur on occasions when translating), and anyone reading the contract under normal circumstances would automatically understand exactly which pronoun represents which party.
This usually becomes obvious when you take a step back from the document.