Database
[{"myLocalizedField":{"en":"Hello world","de":"Hallo Welt","cz":"Ahoj světe","es":"Hola Mundo"}},{"myLocalizedField":{"en":"There is no one who loves pain itself, who seeks after it and wants to have it, simply because it is pain...","de":"Da ist niemand der den Schmerz an sich liebt, der danach sucht und ihn haben möchte, einfach weil es Schmerz ist...","cz":"Nikdo nemiluje bolest samu, nevyhledává ji, ani ji nechce získat, protože bolest...","es":"No hay nadie que ame el dolor mismo, que lo busque, lo encuentre y lo quiera, simplemente porque es el dolor."}}]
Query
db.collection.aggregate([{$set:{myLocalizedField:{$objectToArray:"$myLocalizedField"}}},{$match:{"myLocalizedField.v":{$regex:"llo"}}},{$set:{myLocalizedField:{$filter:{input:"$myLocalizedField",cond:{$regexMatch:{input:"$$this.v",regex:"llo"}}}}}},{$set:{myLocalizedField:{$arrayToObject:"$myLocalizedField"}}}])