A little background first. My parents were debating whether lemon in bengali was pronounced nebu or lebu. So, I decided to do some research into this and found that in many South Asian languages words that begin with the "L" sound have alternate pronunciations beginning with the "N" sound. For example, lemon can be pronounced as nimbu and limb in Hindi. How did this sort of duality develop historically?
Normally, sounds that are interchanged like this are very closely related. For example, fish in English is related to the the Latin piscus. "F" is closely related to "P". How is "N" closely related to "L"?