One thing I worry about is if the user can directly set `locale` on the object (i.e. mf = new MessageFormat('en-US', "hi"); mf.locale = 'fr-FR'`).  If so we shouldn't actually store a pre-computed munged locale but should instead calculate it on-demand in `_normalizeCount`.  (Given the hierarchical lookup we intend to add to _normalizeCount this isn't such a big deal).  (Hmmm... we could actually do some caching and only recompute the pluralization function to use if the locale has changed (or is unset).)