Hello siddhant ,I will be happy to answer your travel queries from my experiences 😊
* you won't face much problems there If you know English .almost every people of South know English and even if you face some problems,you can use Google translator or you may take help from any local who knows both the language s and note down some important words and things to say at malayalam . If you are travelling to kerela,I have a small suggestion for you, I just checked online and found that there are some great deals for kerela availlable on tripoto itself, do check this out https://www.tripoto.com/kerela/tour-packages HAPPY TRAVELLING
In Kerala, you wouldn't get stuck for not knowing Malayalam. You can easily manage with English. Places like Varkala & Alleppey is filled with foreigners, they obviously manage with English some cases even not that.
If you are really concerned go via tour agents.
People in South prefer speaking their mother tongue only, it's hard for the traveller's to communicate with the local people there, instead of English you can try speaking hindi they will understand much better than English