Database
[{"_id":ObjectId("5c05984246a0201286d4b57a"),f:"x","_a":[{"_onlineStore":{}},{"_p":[{"pid":1,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"En"}]},"c":{"t":[{id:3,lang:"en"},{id:4,lang:"En"},{id:5,"dateP":"20300-09-23"}]}},h:"Some data"}]},{"_p":[{"pid":2,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"en"}]},"c":{"t":[{id:3,"dateP":"20300-09-22",lang:"EN"},{id:4,"dateP":"20300-09-23",lang:"en"},{id:5,"dateP":"20300-09-23"}]}},h:"Other"}]},{x:2}]},{"_id":ObjectId("5c05984246a0201286d4b572"),f:"x","_a":[{"_onlineStore":{}},{"_p":[{"pid":1,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"En"}]},"c":{"t":[]}},h:"Some data"}]},{"_p":[{"pid":2,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"en"}]},"c":{"t":[]}},h:"Other"}]},{x:2}]},{"_id":ObjectId("5c05984246a0201286d4b57b"),f:"x","_a":[{"_onlineStore":{}},{"_p":[{"pid":1,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"En"}]},"c":{"t":[{id:3,lang:"en"},{id:4,lang:"RU"},{id:5,"dateP":"20300-09-23"}]}},h:"Some data"}]},{"_p":[{"pid":2,"s":{"a":{"t":[{id:1,"dateP":"20200-09-20",lang:"EN"},{id:2,"dateP":"20200-09-20",lang:"en"}]},"c":{"t":[{id:3,"dateP":"20300-09-22",lang:"EN"},{id:4,"dateP":"20300-09-23",lang:"en"},{id:5,"dateP":"20300-09-23"}]}},h:"Other"}]},{x:2}]}]
Query
db.collection.aggregate([{$match:{"_a._p.s.c.t":{$elemMatch:{lang:{$in:["EN","En","en"]}}}}},{$project:{res:{$reduce:{input:"$_a._p.s.c.t",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}},{$project:{res:{$reduce:{input:"$res",initialValue:0,in:{$sum:["$$value",{$size:{$filter:{input:"$$this",as:"inner",cond:{$in:["$$inner.lang",["EN","En","en"]]}}}}]}}}}},{$group:{_id:0,count:{$sum:"$res"}}}])