Proposed resolution:

In [locale] replace paragraph 6

An instance of locale is immutable; once a facet reference is obtained from it, that reference remains usable as long as the locale value itself exists.

with

Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.

Paragraph 6, says "An instance of locale is immutable; once a facet reference is obtained from it, ...". This has caused some confusion, because locale variables are manifestly assignable.