In this approach, the linguist attempts to acquire proficiency in the target language in order to communicate directly with native speakers.
"[1] The monolingual approach was advocated by linguist-anthropologist Franz Boas, who studied Native American languages,[1][2] in the early 20th century.
Boas believed that gaining command of the language was essential for understanding the culture and worldview of the speech community.
Edward Sapir, one of the best known student's of Boas, used only bilingual approach, even when he worked with Ishi, the last known speaker of the Yana language.
SIL linguists sometimes spend years learning the languages in distant communities, with the goal to learn a language on a level needed to translate the Bible; SIL linguists "embraced, even insisted on, the challenges of monolingual fieldwork".