If explicit R_ENCODING_LOCALES setting doesn't exist tools:::.check_package_code_syntax() uses locales that don't exist on OS X (e.g., en_US.utf8). Ideally, it should use locales that do exist for OS X builds.
Ideally it would for all other OSes too! However, the manual says
it is strongly recommended to check in a UTF-8 locale.
and this value is not used if that recommendation is followed.
I've changed the UTF-8 one to the more common .UTF-8 in R-devel.