The AMAZON.DURATION slot type returns ISO duration. That's fine, but if you use the slot in a dialog, Alexa echos the letters rather than re-interpreting the value back to conversational language.
Example: PT1H15M should be spoken as '1 hour, 15 minutes' instead of 'pee tee one ach one five em'.
This way the slot type can be used in delegated dialogs. This will need to be done for each of the supported languages.