I've never been polled on which languages I speak fluently, so I would doubt that information is presently being tracked.
Reviewing locations that are listed on a record (birthplace, mission, endowment/sealing, etc.) are not perfect, but will probably yield the best clues.
For example, many who serve in (e.g. California San Diego Mission) are called to speak English, but many also are called to speak Spanish.
I know of a Brother who was called to serve in Mexico City, but due to an injury early in his mission, was returned home and never actually learned Spanish; twenty years later, he is always having to explain that he doesn't actually speak Spanish when being extended ward/stake callings that require fluent Spanish.