Configuration entities save in site default language by default

The original introduction of configuration entities made them use the 'und' language for initial configuration entity storage. However most configuration entities in core contain some kind of human understandable label or description, and therefore are inherently in need of language associations. The configuration system is aware of language associations on the entity level (as well as standalone .yml file/configuration name level), so saving the configuration entities in the right language is important.

Some configuration entities have direct language selectors on them, like vocabularies and views, while not all of them make sense to have a user interface element for this. The built-in assumption therefore is that new configuration entities created without an explicit language defined will save in the site's default configured language. That is, if you are running a primarily Spanish site, you are likely to create content types, contact forms, menus, etc. in Spanish. Drupal 8 contributed modules such as http://drupal.org/project/config_translation rely on this information to let you translate configuration to other languages as appropriate.