Can I appeal the costs of a trial?

I have lost a court proceeding about the sale of a property. In the judgment, the judge establishes that I am obliged to also pay the court costs that the plaintiff has had to pay.

Am I entitled to appeal against the amount that they are claiming, if I consider that these costs are excessive?

Yes, you are. The other party will have to prove all the expenses that they have paid (lawyer, translators, experts, etc.,…), providing detailed invoices. Moreover, those professionals who have to be members of a professional association in order to be able to perform their job (such as lawyers, for example), cannot charge whatever amount they want or the amount that they could have freely agreed with their clients, but they have to calculate their fees according to the rates officially established by the professional association.