Year might be enough, or not. It strongly depends on which language you are learning, how different is it from the language(s) you already know, and what kind of proficiency level you want. And process of learning language will be very different from the way we learn new language now.
Big problem you will have when learning new language is that pencils and notebooks and dictionaries were not invented yet. You have to remember it all, cannot write any notes which you can conveniently take along and consult/review. Which will be a problem if you are visual learner - prefer visual input over audio.
Just take audio language course, like Pimsleur, and check how many times you would prefer to read what was said to see the differences. Even if Pimsleur translates it all for you.
Some tonal languages have up to 8 tones for every syllable - and different tones have different meaning (syllable "ma" repeated 5 times in different pitch means "crazy horse run though village"). So unless you have a musical ear (experience of playing instrument, like piano), and expect that, you will have hard time distinguishing differences. Some click languages have up to 100 consonants you may have hard time to reproduce.
Some mental concepts can be completely different. I recall that some Australian tribe has no concept of "left" and "right", but only "north" and "south" - like "there is ant next to your north leg". So you may need to change your mental model what language is about.
Child's brain has advantage of neuroplasticity. Adult brain is substantially less flexible, and has harder time to adapt to new grammar. According to some research, by 12 is too late to start learning second language - you will never become like native.
Also, nouns are easy, you can point and ask. Verbs are much harder. Verb persons and tenses and moods are even more complicated. Some verb tenses might not be present in your mother language. If you know Spanish - imagine if most verbs were irregular. You would have hard time even to see they are related. It's not like in English, where all persons are the same (or adds 's).
Spanish and English are similar languages from Indo-European languages family. Learning Spanish in few months says very little about how much time you need to learn some more obscure Chinese dialect, or click language like Xhosa. In example, even experienced Japanese learners of English have problem distinguishing R from L - because that difference does not exist in their mother language.
In many languages, there is gender in verbs. Your (male) explorer will make fun of himself if will be using female form of verbs, learned from farmer's wife and daughters (which might be assigned to help him, because he will be able to do only simple works to earn his food).
If the only language you know is English, and the only other one you tried to learn is Spanish, you have no idea how more complex are other more different languages.
Especially if neighbor of your farmer is speaking different dialect, or if you have a bad luck and spend a year to learn some obscure language spoken in a few mountain villages.
Summary: If you are lucky and new language is similar enough to yours, you may be able to function in your new community in a year. If not, you will struggle, be able to speak very limited language (and be handled/helped like a child, and not considered equal or competent ). And likely, you will always speak with an accent.
And because you are considered incompetent (dim-witted: cannot even speak properly), you will ave hard time to "sell" your ideas about more advanced technologies you know are possible, but unable to explain with your limited vocabulary. Or you break some tabu, commit some crime and they decide to abandon you, as not worth taking care of.
How I know it: I am fluent in two more languages beyond my mother language (which is not English), and have basic understanding of few more. Even after 20 years of "full immersion" into English I do have accent and make mistakes. And as far as languages go, English is rather easy.