/u/TheLegendofNick

In the specific case of Francis Xavier, the first Christian (Catholic) missionary to Japan, language was a significant barrier. Through is travels in South Asia, Southeast Asia, and East Asia, Francis Xavier knew that language was important, so he sought to learn local languages to varying degrees of success.

In the specific case of Japan, he was fortunate to have run into a Japanese samurai, Anjirō, in Southeast Asia. Anjirō had left his home town in Satsuma following a murder, and he was interested in Christianity from his interactions with Portuguese sailors in East Asia. So he came to Malacca to seek out Francis Xavier, during a period when St. Francis was in the Moluccas. Disappointed, Anjirō was on his way back to Japan when a Portuguese captain persuaded him to turn back to Malacca. They finally met in 1547, leading to Anjirō's conversion to Catholicism when the two traveled to Goa together, at that time the site of the Jesuits' administration in Asia.

Finally, the two traveled back to Japan with three other Jesuit missionaries, arriving in 1549, hosted by Anjirō's family as St. Francis started his missionary work in Japan. By their account, up to 1551 the Europeans were still not fluent in Japanese and had to rely on translated tracts to aid in their preaching, and on translators. Thus, Anjirō played an important role in evangelization as the Jesuits relied on his translation. This was also the time when St. Francis and the Jesuits started to specifically adapt their missionary work to the existant cultures of the countries they work in, rather than the previous approach of tabula rasa that more emphasized acculturation to European culture.

One outcome of that translation effort was to use the word Dainichi for the Christian single God, which they did not realize in Japanese meant the Vairocana Buddha. It was only in mid-1551 that they realized this significance, and started to use the word Deus instead.

Language was a major obstacle, and one that missionaries took seriously.