In the mid 1960s, when the Indian Navy began to acquire equipment from Russia, there was hardly anyone within the service who knew the Russian language. That began to change as India increased military purchases from Russia. By 1970, many Indian naval personnel mainly from the 25th Missile Squadron were undergoing naval training in Russia (Then USSR).
The 25th Missile Squadron was based at Mumbai and equipped with brand new Russian Osa class missile boats armed with Styx missiles. In the western theater in December 1971, the Indian navy planned to hit Karachi on the very first day that Pakistan attacked India.
OPERATION TRIDENT :
War broke out & Pakistan Air Force jets attacked Indian airfields on the evening of December 3, 1971. The Indian Navy decided to attack on the night of December 4. In order to inflict maximum damage and to confuse the enemy, the raid was to be coordinated with a strike by the Indian Air Force on Karachi harbour.
A key and unusual advantage with the Indian Navy was the Osa class missile boat crew’s fluency in the Russian language. Communication between the attacking vessels, Naval HQ at Mumbai and the IAF was extensively in Russian.
This was done to fool the Pakistani naval intelligence before the commencement, and during the attack. The enemy could not connect the chatter on the radio waves to any offensive sea movements by the Indian Navy.
In fact, Pakistani intelligence believed the radio chatter came from the Russian Navy’s detachment located further south in the Arabian Sea. They thought the Russians were manoeuvring in response to the US Navy’s movements in the region.
The operation was conducted on the night of 4th–5th December 1971 and inflicted heavy damage on Pakistani vessels and facilities. India celebrates itsannually on 4 December to mark this operation.
The Indian Navy’s use of Russian language to communicate during the 1971 strike on Karachi harbour proved crucial in the success of the operation.
This has been taken from Quora by an answer by Vijaypal Singh.