This expectation may be formalized through a degree requirement in your program to learn a second language. Or, it may be an unspoken norm, as many senior academics see overseas travel and language acquisition as a sign of true scholarship. Learning a second language may also be a political choice. For instance, meetings of the Canadian Anthropology Society (CASCA) over the past few years have raised the question of the politics of language in (Canadian and World) Anthropology/ies and the dominance of English-language scholarship (and the AAA). Learning another language may be part of your professional development strategy (even if you study a site where you use your first language). In Canada, for example, federal public servants are usually required to have competency in both French and English. Meanwhile, the market research companies that are increasingly hiring ethnographers may be interested in candidates with specific language skills.
In any case, linguist Stephen Krashen has some important advice regarding how to approach language learning in this piece from The Washington Post: "The wrong and right way to learn a foreign language" (2012). Bascially, Krashen advises that:
"We do not master languages by hard study and memorization, or by producing it. Rather, we acquire language when we understand what people tell us and what we read, when we get “comprehensible input.” As we get comprehensible input through listening and reading, we acquire (or “absorb”) the grammar and vocabulary of the second language."