Latin originates from the ancient province of Latium in Italy, which includes the city of Rome. With the growth of the Roman Republic and then the Roman Empire, Latin became the lingua franca of the Mediterranean Sea. Latin pushed out and replaced the indigenous European languages, especially the Celtic ones, during the Roman Empire's height. After the fall of the Empire, Latin progressively vanished as a spoken language except to act as a language for academia and the Roman Catholic Church.
Latin has had an indelible mark on the history of Europe. The Romance Languages - Italian, Portuguese, Spanish, French, Romanian, Catalan, and a handful of lesser-known languages, each generally spoken in only one island or province within a larger nation - are all descended from Latin. Latin was critical to science and literature on a continental scale in Europe until it was replaced by French in the 17th century. Latin still maintains a position within science, for example in the naming of species. Latin also maintains a central position within the Roman Catholic faith.
In Agrippan Rome, daily life was conducted in neoLatin, a language which developed out of Classical Latin during the Empire's more than two thousand years of existence. Its grammar was simpler, dropping most of the cases which are characteristic of Classical Latin - parallel with with the development of Latin-descended languages in the Home timeline. Classical Latin was still extensively used as the language of the government administration as well as in literature. Knowledge of Classical Latin was a major status symbol marking out the Empire's rich.
Knowledge of Classical Latin, gained through the Home timeline's implants was a major asset for Jeremy Solters and his sister Amanda when suddenly stranded in besieged Polisso. Their perfect knowledge of Latin marked them as members of the elite, and forced local rich people to take them seriously despite their youth. When challenged by Roman government agents to produce a report about the origin of the goods which they were selling, which actually came from the Home timeline, Jeremy used the ambiguities of Latin grammar to compose an impressive-looking report which contained no real information.
The Federated Commonwealths of America, a nation which modeled itself on the Roman Republic, used Latin sayings on monuments, government buildings, and plenty of courtroom terminology. It was not the common language of people in the FCA, however. The humans mostly spoke English, as their ancestors had come primarily from England, and the sims spoke no language at all but used a system of hand gestures instead.
When Cade Curtis was surprised by three North Koreans, while fleeing the disaster by Chosin Reservoir, he despaired and began saying the Ave Maria in Latin. To his astonishment, the three crossed themselves and one continued the prayer also in Latin. Christians of all denominations were persecuted by Kim Il-sung's administration so they assisted in Curtis getting back to American lines. Small groups would pass him along from one to the next, giving him shelter during daylight hours and food. Curtis could speak with Catholics, after a fashion, in Latin but made do with sign language with Protestants.
When 20th Century attorney Nicole Gunther was transported back to the 2nd-Century Roman Empire, the gods Liber and Libera provided her with a complete knowledge of the Latin language, including even literacy in it (which Umma, whose body Gunther inhabited, did not have). When later coming in contact with the Emperor Marcus Aurelius and his aides, she realized that the Latin with which she had been provided was a provincial dialect in which the final consonants were not voiced (anticipating the later development of French and other Latin-descended languages) while members of the Imperial Court spoke a far more refined and "pure" form of Latin.
On returning to the Twentieth Century and Los Angeles, Nicole retained her knowledge of Latin. She had little practical use for it, but it helped convince her that her time in the Roman Empire had been a reality and not an illusion.
Gordon McSweeney, a soldier in the United States Army during the Great War, was a Presbyterian who had an obsessive bigotry against most other Christian denominations, and often spoke about the evils of Roman Catholicism. This phobia could be easily aroused by hearing the Latin language, and he frequently advised anyone who would listen about the iniquities perpetrated in that tongue. On one occasion he heard another soldier in his unit speaking in what he perceived to be Latin, but it turned out the man was speaking Greek not Latin.