I downloaded this “dummie’s guide” for Sugbuanon (Cebuano). It was written by and for Mormon missionaries. The Mormons are brilliant linguists, in practice. One time a 23-ish Mormon missionary from Utah came up to me in Lumphini Park in Bangkok and started talking to me in Thai. I didn’t know Thai back then, so he switched to Mandarin. He was fluent, with good pronunciation. He’d been posted on Taiwan for like seven or eight months, once upon a time.
Other folks hack languages to get paid, to get laid (dudes), or to trade up, caste-wise (chicks). For Mormon missionaries, it’s none of the above — yet they be doing it better than all of us put together.