A marvelous stroke of luck, that's how.

In 1511, a Franciscan friar named Gerónimo de Aguilar and a bunch of companions were captured by the Yucatec Maya. Some of the companions were sacrificed or overworked as slaves, but Aguilar and another man escaped and were taken in by another tribe, where they were a bit better treated. Eight years later, Cortez encountered this tribe and took in Agular, who became a huge asset to him, as he was fluent in the local language, which we call Yucatec.

Later, when Cortez' expedition encountered the Maya city of Potonchán in the modern state of Tabasco, he faced hostility from the locals and fought with them. He won, and the Maya sued for peace and gave him an offering that included a number of slaves, including a young woman who would later be known as La Malinche.

La Malinche, or Marina, or Malintzin (we only know her by these names, and not her original one) was apparently a Nahua woman of noble birth who was sold into slavery at a young age (around 10), possibly to pay for her family's debts, which was unfortunately relatively common at the time. Being the daughter of nobility (if not kings), meant she could understand the higher registers of Nahuatl speech, which was a bit different from the speech of commoners, though she likely learned to speak this as well. Nahuatl, by the way, was both the language of the Mexica/Aztecs, and generally the lingua franca of trade and diplomacy in much of Mesoamerica at this time.

Anyway, La Malinche seems to have been something of a linguistic prodigy, and she had apparently picked up some of the local Mayan languages, including Yucatec, which Aguilar had learned during his long captivity. Soon, the Spaniards realized could now use the two of them as a chain of translation to speak to almost anyone they needed to. Within a few short years La Malinche seems to have mastered Spanish as well, and eventually Agular was no longer needed, which definitely facilitated easier communication and more freedom for her to express her diplomatic skills. By the time the Spaniards were in Tenochtitlan, La Malinche seems to have been speaking directly to Moctezuma on Cortez' behalf.

La Malinche's role in the Conquest cannot be overstated. Her value not only as an interpreter, but apparently as a brilliant diplomat, helped the Spaniards build the army that dismantled the Triple Alliance. This was understood very well at the time, and is reflected in the primary sources. Native codices describing the conquest depict her side by side with Cortez, and often exceeding him in size and splendor. Conquistadors who wrote about the events they witnessed praise her endlessly, and one even says that "after God, Marina was the main reason the Spanish succeeded."After the Conquest her life is poorly documented, but it is known that she had a son with Cortez named Don Martin (who was illegitimate, but legitimized through a Papal bull), joined Cortez on some other expeditions, and later married a Spaniard named Juan Jaramillo, with whom she had a daughter. Unfortunately it isn't known how or when she died.

Anyway, to make a long story short, had the Spaniards not found La Malinche, everything they had done would have been more difficult. Maybe they could have found another interpreter who knew both Yucatec and Nahuatl, but could they have found one who was also highborn and spoke the upper registers of nobility? And if they were able to find someone like that, would this person also be quick-witted and subtle enough to be skillful at diplomacy? La Malinche seems to have been all of these things, so it was incredibly lucky they found her, and it's probably not going too far to say that the Conquest wouldn't have succeeded without her help.