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