This takes effect when an interpolation is removed from a translation in
a Discourse update.
The I18n::Backend::Fallbacks loops with a catch(:exception), so calling
throw(:exception) will cause it to use the next locale, until it reaches
English which is assumed to be correct.
Also, enable fallbacks in everything except development (#3724 for more
discussion) - we should be able to test this
The FallbackLocaleList object tells I18n::Backend::Fallbacks what order the
languages should be attempted in. Because of the translate_accelerator patch,
the SiteSetting.default_locale is *not* guaranteed to be fully loaded after the
server starts, so a call to ensure_loaded! is added after the locale is set for
the current user.
The declarations of config.i18n.fallbacks = true in the environment files were
actually garbage, because the I18n.default_locale was
SiteSetting.default_locale, so there was nothing to fall back to. *derp*
