Short Answer- Bilingual individuals. Before modern times is a very broad category, but bilingualism seems to have been very common prior to the modern era, perhaps even more common than nowadays.

Bilingual individuals were incredibly useful in the context of warfare in particular and the ability to speak other languages is often considered a valuable attribute. There were multiple ways that individuals could become bi- or often tri-lingual. One is growing up in a contact zone.

An example here would be Tancred of Hauteville, an Italo-Norman who participated in the first crusade and was an early ruler of the Principality of Antioch. Crusade histories seem to indicate that Tancred spoke, at least, the Norman dialect of Old French, Greek, and Arabic.

This is not surprising given that he'd grown up in Southern Italy during the Norman conquest of the region. The conquest came at the detriment of Byzantine and Muslim territories and Greek and Arabic remained important languages in the region for long after the conquest.

Tancred's ability to communicate directly with Greek-speaking Byzantines and Arabic-speaking Levantine powers as well as other groups that used these as lingua francas, such as the Armenians and Turks, was no doubt very useful during and after the crusade.

Another practice that often led to bilingual individuals was the trading of hostages, which was very common in antiquity. One famous figure from Roman history is Arminius, the son of a Germanic chieftain, Arminius and his brother were handed over to the Romans and learned Latin from a young age.

This practice sought to enable closer ties between the Roman Empire and its neighbors, Arminius and others like him would, ideally, act as a bridge between Roman administrators and locals. In the case of Arminius, this did not turn out so well for the Romans.

The need for interpreters also came into sharp relief during the early years of European colonialism and similar to how the Romans had used hostages as intepreters, captives of various kinds were very useful.

One famous example is la Malinche. Malinche was a Nahua woman, likely of noble background, who ended up enslaved by a group of Chontal Maya. She then learned the Chontal Maya language. Around 1519, Malinche was among a group of enslaved women given as a peace offering to a Spanish expédition led by Hernán Cortés. She was baptized Marina and this was later translated into Nahuatl as Malintzin and then back into Spanish as Malinche.

However, when Malinche was handed over to the Spanish, Cortés already had an interpreter, a Spanish friar by the name of Gerónimo de Aguilar. Gerónimo had traveled to the Yucatán around 1511, but after a shipwreck, he had been captured and presented to a Mayan chief alongside a sailor named Gonzalo Guerrero. The two spent around 8 years with the Maya and learned their language.

When Cortés arrived, he heard rumor of bearded men working with the Maya. He reached out to them and they were indeed the Spaniards Gerónimo and Gonzalo. Gerónimo joined Cortés, Gonzalo, however, was reluctant to leave his position as Maya war chief, as well as his Maya wife and Mestizo children. He would later die fighting his former countrymen.

Gerónimo, however, joined Cortés and was able to translate from Spanish to Yucatán Maya. However, the expedition ran into trouble when they came into contact with emissaries from Moctezuma Xocoyotzin, Ruler of Tenochtitlan and Emperor of the Triple Alliance, also known as the Aztec Empire.

The Aztecs did not speak Maya, they spoke Nahuatl. However, Malinche spoke both Nahuatl and Maya, thus, she could translate Nahutal to Maya for Gerónimo who could then translate Maya to Spanish. Malinche later learned Spanish and simplified the process, the Spanish chronicles refer to her as Doña Marina la lengua, Lady Marina the Tongue, in honor of her role as key interpreter.

These are just a few examples, but you can find other similar stories everywhere linguistically diverse groups of people come into contact. Translation was done by anyone who'd had the chance to learn both languages, when contact was newly established, captives often shined as testaments to just how quickly one can pick up a language when circumstances are dire.