In Argentina several laws are sanctioned which later are proved to be unconstitutional. Yet, until someone appeals the law, that law is applied unconstitutionally to a lot of people with the harm it makes to them. Many times you know that a law they are about to approve is wrong, it's approved by legislators not for the sake of people but to profit their own interests, and that the law is going to harm many people. My question is: Can such legislators be sued?
Short answer, no.
Articles 68-70 of Argentina's constitution effectively provide for legislative immunity while in office.
There are two issues with your plans to sue: 68 protects the legislature for opinions and speeches (such as approval of an unconstitutional law).68 appears particularly troublesome to your lawsuit, as it covers conduct that occurred during their office holding tenure, and seems to continue after their tenure. Note: I have not read this in Spanish, and instead I am reliant on the below translation.
Now, if your accusations are true, you could make a case for this to be considered fraud or treason (or some other crime). If that legislator is currently in office, then their respective House would have to suspend them before any investigation could continue (which is unlikely). Also, regardless of during or after their term, you'd have to convince a prosecutor to press these charges, as these would be criminal charges, and those are (generally) the state's sole prerogative to prosecute.
Section 68.- No member of Congress shall be accused, judicially examined, or disturbed for opinions expressed or speeches delivered by him while holding office as legislator.
Section 69.- No senator or deputy shall be arrested as from the day of his election until the expiration of his term, except when flagrantly surprised committing a crime deserving capital punishment or other infamous or serious punishment, in which case a summary report of the facts shall be submitted to the corresponding House.
Section 70.- When a written complaint is filed before the ordinary courts against any senator or deputy, once examined if there is enough evidence in a public trial, each House may, with the concurrence of two-thirds of the votes, suspend the accused party from his office and place him under the jurisdiction of the competent court to be judged.