I don't understand how Alexa handles unmatched utterances. There is a AMAZON.FallbackIntent available only in English and French. So for my French skills, after submission they reported :
While interacting with your skill we determined that, in certain cases, your skill does not respond with a clear and audible response to some of the inputs provided. We do not allow skills to respond in an inaudible or unclear manner to customers.
Upon providing long irrelevant input to the skill in an open session, the skill should end the session post 8 seconds or provide a verbal prompt understanding that the audio input is irrelevant. Expected Behavior: Your skill should respond in an audible and clear manner to all customer input by providing instructions on what to do next.
I don't understand how to handle this feedback
UPDATE: FallbackIntent is now also available in French, Italian, Spanish and Japanese
AMAZON.FallbackIntent is only available for English and German locales. The certification feedback means that you're not handling out-of-domain utterances gracefully (this can happen even on skills where the locale supports the fallback intent).
If the feedback in question is for the French version of your skill (i.e. you're handling this correctly in the English version thanks to the fallback intent availability) your only option until fallback become available in other locales is to follow the advice I'm giving here:
How to fallback into error when reprompt is processed in Alexa Skill