I see many folks throw the triggering word "mandatory" which is false and it's not my opinion but fact. had shared the document some time back on this very issue.
Let's say that Hindi is not mandatory of the language policy that BJP government brought back but apart from the schools, just look at what government is trying which is Hindi is the national language of India whenever they comes before the media that triggers the non Hindi speakers especially Kannadigas and engage in violent acts because that's how they survive in their political game and hide away from the real problems so the intention of playing this language game is to distract people from real issues.
Why "Let's say that Hindi is not mandatory of the language policy"? It's not some hypothetical situation but official document of Gov, one can not just assume anything when it's not even mentioned in any policy remotely.
Southern states and west bengal has concern but it's political and not cultural. In my understanding they just assume and scared that if this policy is implemented then everyone will automatically opt for hindi as their 3rd language because its a logical choice for keralite, tamilian etc to learn Hindi due to its reach instead of other Indian language.
It's funny that kannadigas screaming the most, considering Hindi imposition was congress's brainchild and they made it mandatory to learn Hindi as 2nd language and English as 3rd language. While BJP is saying, first learn your mother tongue, then English and after that any Indian language as 3rd option.