system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did we know how to translate between different languages.

Let's say for the first time in human history, someone tried to translate something from Chinese into English or vice-versa. How would they know what the right words and sentence structure were in the other language?
gpt
For any living language, it's possible to learn a second language the same way you learned your first. Go to a place where everyone is speaking it and be curious. You'll eventually pick up enough of the basics that a current speaker can start to teach it to you formally. For languages spoken nearby, dual speakers will naturally arise in children who spend some time in each region. Chinese to English may have been harder at one point, but because of the nature of trade in olden times, if you even *had* any Chinese text, you probably also had a Chinese person who had traveled a long way to deliver it, perhaps so long that they had no intention of returning any time soon and could learn English and translate. 

Dead languages are trickier, and European scientists had a famously difficult time translating things like Egyptian hieroglyphics. That problem was addressed by the "Rosetta Stone", a fortunate archaeological find that  printed the same text in multiple languages, one of which we already understood and one of which was hieroglyphics.