San Lorenzo is a republic in Central America, where Arnold's parents, Miles and Stella, worked and explored for many years, until they mysteriously disappeared. Miles and Stella's child, Arnold, was born out in an abandoned temple in this region. Abner most likely was born in San Lorenzo too.
Many glimpses at San Lorenzo are given in the episode "The Journal", in which Arnold finds his father's journal and reads about his adventures in the country. In the end of the episode Arnold finds a map of the region of San Lorenzo where his parents went in their last journey. The main events of Hey Arnold! The Jungle Movie take place in in this country.
The shown areas of San Lorenzo are covered by jungles, they have at least one active volcano, at least one navigable river (roamed by the river pirate La Sombra), and remains of ancient Mayan-like architecture. Considering the combination of these factors, San Lorenzo most likely corresponds to Guatemala and/or neighboring areas of Mexico and Honduras. The area is homeland to mysterious people who are known as the Green Eyes. Other ethnic groups include the Spanish-speaking people (one of them is Eduardo) and Amerindians who speak a gibberish language. A large body of water was shown, but it is not clear whether it is a sea or a large lake.
San Lorenzo is also the name of an insular nation described in Kurt Vonnegut's book Cat's Cradle. Craig Bartlett confirmed on his second interview on The Arun Mehta Show that the coincidence of the names is indeed intentional.