Localization of messages

Messages that IdentityServer creates can be localized (or simply replaced) by implementing the ILocalizationService interface. It defines a single API:

This API is passed a category for the message. IdentityServer only defines three categories:

public static class LocalizationCategories
    public const string Messages = "Messages";
    public const string Events = "Events";
    public const string Scopes = "Scopes";

The id parameter then indicates the specific message from that category. The various identifiers that would be used are defined by constants in the IdentityServer3.Core.Resources namespace. For example, here is a short snippet of those constants (consult the code for the full list):

namespace IdentityServer3.Core.Resources
	public class MessageIds
			public const string ClientIdRequired = "ClientIdRequired";
			public const string ExternalProviderError = "ExternalProviderError";
			public const string Invalid_scope = "Invalid_scope";
			public const string InvalidUsernameOrPassword = "InvalidUsernameOrPassword";
			public const string MissingClientId = "MissingClientId";
			// ...

The default implementation of the ILocalizationService loads all of its messages from embedded resource files (“.resx”) contained within IdentityServer itself. These embedded resource files only contain English strings.