Fix inflector to respect default locale.

The inflector was made aware of locales in 7db0b073fec6bc3e6f213b58c76e7f43fcc2ab97,
but it defaulted to :en. That should actually be our default
locale instead.

Fixes #10125

I wanted to get some feedback about the tests. Is this the right place, or should I bet testing singularize/pluralize more directly?