This patch sets I18n.defaultLocale in the Discourse.start() script block (it
was formerly always 'en') to SiteSetting.default_locale, and patches
translate() to perform fallback to defaultLocale followed by english.
Additionally, when enable_verbose_localization() is called, no fallbacks will
be performed.
It also memoizes the file loading operations in JsLocaleHelper and strips out
translations from the fallbacks that are also present in a prefered language,
to minimize file size.
728845d008