Receive all locales with ElementAPI plugin
Here are some other similar questions and answers to this, but the short of it is that you could probably do it with something like using QueryBuilder to query the database directly (and bypassing Craft's APIs) or grabbing all site locales and looping through and re-querying for the entries on a per locale basis.

Both of them are pretty ugly and probably not that efficient in the context of the ElementAPI plugin.