What does an expert translator do? They are trained and certified professionals in the field of translation who are responsible for testifying in court about any controversy that can arise surrounding a translation. They work as an assistant to the court and can intervene either to translate or to iron out conflicts that have their origin in the poor quality of a translated document. The expert can be officially designated. This means that they can be convened by the parties or by the judge, as the case may be.
In Argentina, which is the country I have the most experience with, the controlling body for the actions of experts in translation is the Association of Public Translators of the City of Buenos Aires (Colegio de Traductores Públicos de la Ciudad de Buenos Aires). Any translator in Argentina who is looking to work as an expert should first be registered in that institute. Then they must take a course there that authorizes translators to enroll as experts and lasts a few months. Please make sure that, if you are looking to work as an expert in a different country, you do the right research to find the correct organization for your country.
Lastly, it is worth noting that the work of an expert implies the highest responsibility in their work, since the translator works as a consultant for the judge and this means working with the utmost decency, professionalism, and honesty.