The changes are proposed in #615 .

The changes involved:

• MessageCatalog interface exposes two funcs that can be used to choose an implementation of choice. The DefaultMessageCatalog provides a simple implementation.
• By default, MessageCatalog is nil and the implementation falls back to current behavior (tested by running the unit tests)
• WithHint and WithHintf record the original formatted message and arguments for legacy translation. Here the catalog contains a message ID that matches the formatted message.
• WithHintIDOrDefaultf allows a message ID to be specified along with a default message. If a localizer is provided, the message ID is used to find the message and the vaargs are applied (if present). Otherwise the default formatted message is used.
• Requesters have been updated to implement a G11NContext interface that declares a GetLang func.

There are no Breaking Changes.

NOTE: Some gaps exist in this implementation with the introspection handler because of a missing requester object. This will be addressed in a future PR.

Related issue(s)

Linked to PR.

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

Further comments

The actual translation of messages is left to the point when GetDescription is called. Thus, the localizer only needs to be known to the error object at this point. This allows us to limit the changes to when error responses are built. For example, when WriteAuthorizeError is invoked.

In both the authorize and token flows, the requester object is passed into the Write*Error func making it simple to extend the requester object to implement the G11NContext interface that declares a GetLang function. With this, all that a consumer needs to do is add a MessageCatalog instance to the compose.Config and the changes here take care of the rest.

However, WriteIntrospectionError does not accept the requester and, so, a helper function - AddLocalizerToErrWithLang - is provided to augment the error object with the localizer details (catalog and lang).

For example:

        tt, ar, err := oauth2Provider.IntrospectToken(ctx, token, fosite.TokenType(tokenType),
		session, strings.Split(scope, " ")...)
	if err != nil {
		newerr := fosite.AddLocalizerToErrWithLang(h.s.MessageCatalog(), i18n.GetLangFromRequest(h.s.MessageCatalog(), r), fosite.ErrRequestUnauthorized.WithHintID(i18n.ErrHintInvalidHTTPAuthzHeader))

		oauth2Provider.WriteIntrospectionError(rw, newerr)
		return
	}

This is somewhat cumbersome and should probably be enhanced to pass in the requester to the WriteIntrospectError. In the interest of ensuring backward compatibility, I have chosen not to make these changes but an overload func WriteIntrospectErrorWithRequester can be considered in a future update.