Found via `codespell -q 3 --skip="./app/javascript/mastodon/locales,./config/locales"`

Previously the default locale was set by Localized concern for controllers,
but it was not enforced for mailers.

config is enforced throughout the application and an appropriate place to
set the default locale.

* Use I18n.locale instead of ":en"
* Reset I18n.locale value after locale changing tests

* Improve default language decision

This change allows to takes account of accepted language determined by
the user agent even if the custom default locale of the instance is
configured.

* Cover Localized more

* Fix code style