Non-portable Apple MacOS specific predicates are prefixed woth
apple_.
en for
english followed by an undercore and an identifier for the Region
in the MacOS
Language & Region preferences. For example, with the
primary language set to “English (UK)” and the Region
to “United Kingdom” we get en_GB. This relates
to the locale identifier
en_GB.UTF-8. Unfortunately it is not that simple. For
example, we can combine the primary language “English (UK)” with
the
Region “Netherlands” to end up with en_NL
which is not a valid MacOS locale.