That is because the LDS church believes every person should hear the Gospel in his or her own language. Teachers are usually returned missionaries. Often, in the early going, the spirit is willing, but the tongue won't cooperate. Mormons also believe Jesus appeared again after his Resurrection, this time to an ancient civilization in the New World. The record of that event is contained in the Book of Mormon, which they believe is divinely inspired, like the Bible.
Training is sophisticated.
Computers help students master pronunciation. The young missionaries are taped as they practice presenting their message to native speakers. The early going is painfully difficult. From now on, the missionaries get one personal day a week. Most use it to keep up the rigid personal appearance standards the church demands.