u/Total_Markage and I have written about this in the past:

Was there much contact between the Crusader states and the Mongol empire?

What was happening in Europe, north Africa & the middle east during the Mongol invasions

I just read an article on wikipedia that said that the papal states came into contact with the mongols in the middle ages and they sent letters to eachother, how could they translate the letters back then?

Total Markage is the new Mongol Empire flair so I'm sure he will have even more information.

But in short, Marco Polo was actually pretty late to the scene, since there had been several European travellers in Asia long before him, including his own father and uncle. He was able to travel so far for so long because much of Asia was actually already pretty well known by that point.

Among the other travellers were Lawrence of Portugal, Giovanni del Pian di Carpini, Andre of Longjumeau, Ascelin of Lombardy, and William of Rubruck. Some of their accounts survive and have been translated. There was also a mission by an Armenian ambassador, Smbat Sparapet.

A few decades later, Marco Polo and his father and uncle were there, as was John of Montecorvino. Then into the 14th century and afterwards, contact became more regular.

Sources:

- Igor de Rachewiltz, Papal Envoys to the Great Khans (London, 1971)

- Jean Richard, La papauté et les missions d’Orient au moyen âge (Rome, 1977) (in French, but still a classic if you have access to it)

- Antti Ruotala, Europeans and Mongols in the Middle of the Thirteenth Century (Helsinki, 2001)

- Peter Jackson, The Mongols and the West (Routledge, 2005)

- David Morgan, The Mongols, 2nd ed. (Blackwell, 2007)

And some English translations of primary sources:

- Peter Jackson, The Mission of Friar William of Rubruck: His Journey to the Court of the Great Khan Möngke, 1253-1255 (Hakluyt Society, 1990)

- Christopher Dawson, The Mongol Mission: Narratives and Letters of the Franciscan Missionaries in Mongolia and China in the Thirteenth and Fourteenth Centuries (New York, 1955)

- Giovanni del Pian di Carpini, The Story of the Mongols, Whom We Call the Tartars, trans. Erik Hildinger (Branden Books, 1996)