The Bohemian Czech language is first recorded in writing in glosses and short notes during the 12th to 13th centuries.
By then the language had developed a literary tradition, and since then it has changed little; journals from that period have no substantial differences from modern standard Czech, and contemporary Czechs can understand them with little difficulty.
In addition, a prothetic v- is added to most words beginning o-, such as votevřít vokno to open the window.
This begs the question: Why did none of them feature prominently in the film? While his claim might sound hyperbolic—or like a canny bit of marketing—it rings true: He logged thousands of miles and hundreds of hours to make the film, braving roach motels, crack highs, and homicidal pimps.