Force all keys of localized strings to be valid ISO 639 two-letter codes

[JsonConverter(typeof(LocalizedStringJsonConverter))]

public sealed partial class LocalizedString : IEquatable<LocalizedString>

{

    /// <summary>

    /// All available ISO 639 Set 1 language codes (and "iv" for the special invariant culture).

    /// </summary>

    public static ImmutableHashSet<string> AvailableIso693Set1Codes = CultureInfo

        .GetCultures(CultureTypes.AllCultures)

        .Select(c => c.TwoLetterISOLanguageName)

        .Where(c => c.Length == 2)

        .ToImmutableHashSet();

    public static readonly LocalizedString Empty = new LocalizedString(

        ImmutableDictionary.CreateRange(

            [

    {

        if (data.Count == 0)

        {

            throw new ArgumentException("Data must contain at least one localized string.");

            throw new ArgumentException("A localized string must contain at least one language variant.");

        }

        foreach (var key in data.Keys)

        {

            if (!AvailableIso693Set1Codes.Contains(key))

            {

                throw new ArgumentException(

                    $"'{key}' is not a valid ISO 693 two-letter language code or the invariant culture code."

                );

            }

        }

        this.data = data;

            return null;

        }

        try

        {

            return LocalizedString.Create((IDictionary<string, string>)dictionary);

        }

        catch (ArgumentException e)

        {

            throw new JsonException(e.Message);

        }

    }

    public override void Write(Utf8JsonWriter writer, LocalizedString value, JsonSerializerOptions options)

    {