Pretty much the same way you learn your first language. You point at something, you say the word for that thing in your language. The other person does the same in their language. You do some action, you say the word for that action. The other person does the same. With some nouns and some verbs you can exchange simple sentences and get some idea of the grammar of the other language.
It’s harder than learning a language from someone who knows both languages, but it’s possible. It’s very rarely needed – if language groups have lived next to each other for a while then you generally have some people who know both.
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