The question asks us to identify which statement does not correctly interpret the language policy outlined in the National Education Policy (NEP) 2020. Let's analyze each option:

1. Statement 1: "Students may change one or more of their three languages in Grade 6 or 7"
   This statement is correct. According to NEP 2020, flexibility is provided regarding language choices in different academic years, allowing students to make changes as needed.
2. Statement 2: "Bilingual teaching methods will be encouraged when textbooks are unavailable in the home language"
   This statement is correct. The NEP 2020 emphasizes the use of bilingual teaching methods to ensure comprehension when materials are not available in the students’ home language or mother tongue.
3. Statement 3: "Medium of instruction will preferably remain the home language at least till Grade 5"
   This statement is accurate. NEP 2020 advocates that, wherever possible, the home language/mother-tongue should be the medium of instruction until at least Grade 5.
4. Statement 4: "The three-language formula mandates Hindi in every region"
   This statement is NOT correct. The NEP 2020 adheres to the three-language formula but does not mandate Hindi in all regions of India. The policy allows flexibility for different states to choose languages based on regional preferences without enforcing Hindi universally.

Based on the analysis, the correct answer is: The three-language formula mandates Hindi in every region, since it inaccurately represents the NEP 2020's language policy.