using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var json = @"{" + 

    "\"Key\": \"ValidateEmail\"," +

    "\"LocalizedValue\":{" + 

        "\"fr-FR\": \"Veuillez fournir une adresse électronique valide.\"," +

        "\"en-GB\": \"Please enter a valid email address.\"," +

        "\"nl-BE\": \"Vul hier een geldig e-mailadres in.\"," +

        "\"de-DE\": \"Geben Sie bitte eine gültige E-Mail-Adresse ein.\"" +

    "}" +

"}";

        var result = System.Text.Json.JsonSerializer.Deserialize<Translations>(json);

        Console.WriteLine(result.Key);

        Console.WriteLine(result.LocalizedValue.Count);

    }

}

​

public class Translations

{

    public string Key { get; set; }

​

    public Dictionary<string, string> LocalizedValue {get;set;}

}