This is to fix what I think was a misunderstanding of the exclude_dflt
keyword (and the implicit include_dflt option active by default).
Rather than including the following lookups in the default language
systems, I think it is used to include the lookups specific to default
language systems in the system defined by the current language
statement. Thus instead of registering a lookup in all default
systems when include_dflt is true, we should exclude the lookups
registered with default systems from the current system when
include_dflt is false.
