Stack Exchange uses Transifex to translate the user interface, but that isn't enabled on a standard Stack Exchange site. A new site must be first be created (for example, https://eo.esperanto.stackexchange.com/), and then the translation is enabled for that site.
Before doing that, Stack Exchange would first look for a person who understand well the language that is going to be used on the site, who will be appointed CM with the task to follow those sites using that language.
At the moment, Stack Exchange doesn't plan to add more translated sites, though. See How can I propose a non-English site?
For more details about localized sites and the use of Transifex for Stack Exchange site, see the questions tagged transifex or localization on Meta Stack Exchange. The list of bugs not yet resolved gives an idea of which issues a site localized in Esperanto would have too.