Ok I like your thought experiment. Lets change it a bit.
Instead of it being an unknown language, its English (a language you know), but every single Noun, Verb, Adjective or Preposition has been changed to Thai (a language you dont know).
The Mæw Nạ̀ng Bn the S̄eụ̄̀x.
If you had sufficient opportunity to study this pile of text, you'd begin to pick out patterns of which words appear together, and what order words often appear in. Suppose you study this giant stack of Thai text for years in isolation. After all this study, you're good enough that given a few written Thai words, you can write sequences of words that are likely to follow, given what you know of these patterns.
Right, and to get good at this task, you'd need to build models in your head. You would think to yourself, right a Mæw tends to nạ̀ng bn a S̄eụ̄̀x, and you would build up a model of the sort of things a Mæw might do, the situations it might be in. In an abstract way. As you absorbed more and more data you would adjust these abstract models to fit the evidence you had.
You dont know what a Mæw is. But if someone asks you about a Mæw, you can talk about how it relates to S̄eụ̄̀x, Plā and H̄nū. You know stuff about Mæw, but its abstract.