* Fix language filter codes

CLD3 returns BCP-47 language identifier, filter settings expect
identifiers in the ISO 639-1 format. Convert between formats,
and exclude duplicate languages from filter choices (zh-CN->zh)

* Fix zh name

* Add failing spec showing that human_locales does not match what i18n knows about

* Add missing `ar` key for arabic to human locales

* Remove duplicate `id` key from available locales

* Sort keys in human locales list

* Add spec for human_locale helper