[{Countries:[[["Russia","USA"],["Egypt"]],[["Japan"]]]}]

db.collection.aggregate([{$project:{"countriesFirstReduce":{$reduce:{input:"$Countries",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}},{$project:{"countriesSecondReduce":{$reduce:{input:"$countriesFirstReduce",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}}])