What is the better way to segregate my customers based on languages they speak? - Topic or Groups? Note: My customers speak multiple Indian languages. I'm not looking for a translation features.

I would go with groups. Because topics are working universally in both community home as well as within groups. Creating private groups for other languages might be a good way to start. Later on, When the community for that language grows, it's better to create a separate instance for that language so it could have its own topics and groups.