The Calm Apple Watch app allows Calm subscribers users to start a Daily Calm session from the watch. For more information and details about downloading apps and using them on your watch, please see this Apple support article.
Setting your Language in the app on your Apple Watch:
At the current moment, the language that is selected within the settings of your apple watch app will be the language available within the Calm app on your watch. Unfortunately, the language selected within your Calm app on your device does not dictate the language set on the app on your watch.