Database
[{description:{fr:"hi",en:"bye",},comment:{fr:"nice",en:"apple",}}]
Query
db.collection.aggregate([{$match:{}},{$addFields:{rootArr:{$objectToArray:"$$ROOT"}}},{$unwind:"$rootArr"},{$match:{"rootArr.v.en":{$exists:true}}},{$group:{_id:"$_id",data:{$push:{k:"$rootArr.k",v:"$rootArr.v.en"}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$data"}}}])