ConfigNamesMapper::getLangcode() should fallback to the site's default language not 'en'

This was originally discussed as part of #2584603: PHP exception on manage fields after enabling Configuration Translation but was then split off. Please credit @claudiu.cristea.

Problem/Motivation

After #2212069: Non-English Drupal sites get default configuration in English, edited in English, originals not actually used if translated the language code of shipped configuration is equal to the site's default language and no longer always en. ConfigNamesMapper contains a fallback for configuration that fails to explicitly provide a language code but it still hardcodes en

Proposed resolution

Fall back to the site's default language instead.

Remaining tasks

User interface changes

API changes

Data model changes