I think you mean
ageCategory = Catégorie d'âge
goodbyeWorld = Au revoir monde
and
MyConstant.properties
is the default file when no matching languages are found.
In your case, this means your application will only run with the French
without the <extend-property...>
MyConstants_fr.properties
will have french translations
and
MyConstants_fr_FR.properties
will have translations that are different than the
MyConstants_fr.properties -
specific to the country of France
MyConstants_fr_CA.properties
will have translations that are different than the
MyConstants_fr.properties -
specific to the country of Canada
So it should read
<!-- French language, sepcifically for France -->
<extend-property name="locale" values="fr_FR"/>