It doesn't seem to me that we've really deprecated getdefaultlocale() -- it's not documented as such, and MAL makes the good point that getdefaultlocale() is trying to avoid calling setlocale().

Perhaps this is just a documentation problem?  _build_localename() just glues together the language code and the encoding, resulting in things like en_US.ISO8859-1, which turns out to not work.  So maybe we should document that getdefaultlocale() will sometimes guess wrong.