Your account's Primary Language is the language in which your flows appear within the flow editor. Note that the Primary Language setting does not dictate the language in which your contacts receive messages. Rather, a contact's preferred language setting dictates the language in which (s)he receives messages.
Adding Additional Languages to your account is the first step in translating your flows. You may add any language that has been assigned an ISO 639-2/B code:
Once you've added Additional Languages to your account, they will be represented within the flow editor beneath the "Start Flow" button and settings menu. Below, the language in black is the Primary Language.
Default Flow Language
The leftmost language displayed beneath the "Start Flow" button and settings menu represents the default language of a flow which, if other than your account's Primary Language, can be changed when creating a flow using the "Language" drop-down menu on the "Create Flow" dialogue:
If the Primary Language of a flow is different from the Primary Language of your account as a result of being changed during flow creation, it will then become the leftmost language displayed beneath the "Start Flow" button and the settings menu:
Click here to learn how to set a contact's primary language preference.