[{"locale":{"en":{"dashboard":{"DASHBOARD_TITLE":"Some title"},"contact":{"CONTACT_TITLE":"Some title"}},"fr":{"dashboard":{"DASHBOARD_TITLE":"Some title french"},"contact":{"CONTACT_TITLE":"Some title FR"}},"pr":{"dashboard":{"DASHBOARD_TITLE":"Some title portugues"},"contact":{"CONTACT_TITLE":"Some title PR"}}}}]

db.collection.aggregate([{"$project":{"locale.fr":{"$arrayToObject":{"$filter":{"input":{"$objectToArray":"$locale.fr"},"as":"el","cond":{$eq:["$$el.k","contact"]}}}}}}])