On our very crowded (and in some parts multi-use) bike path, "on the left" simply means "I'm passing you." The serious bikers, whom I sometimes regard as homicidal maniacs in spandex, are very good about saying it.

I say "on your left" and ring my bell when passing pedestrians, skaters, or bikers. I use the bell because we have lots of tourists and languages/local customs may vary.

If there is some unusual obstruction, like two bikers who stop to chat with one bike parked sideways, I call out "helloooo."