Database
[{description:{fr:"StringFr",en:"TEST",},comment:{fr:"CStringFr",en:"CStringEn",}}]
Query
db.collection.aggregate([{$replaceRoot:{newRoot:{$arrayToObject:{$map:{input:{$objectToArray:"$$ROOT"},in:{k:"$$this.k",v:{$cond:["$$this.v.en",{en:"$$this.v.en"},"$$this.v"]}}}}}}}])