Add mapping target for config entity 'language'

Language added to an entity as a entity reference field is not available as a feed mapping target.

1. Select a bundle from "Home > Administration > Structure"
2. Select "Add field"
3. Select "Reference/Other" as type and save
4. In "Type of item to reference" select "Language" and save
5. Save the other settings with default values
6. Go to a mapping page of a feed that is using the bundle
7. Try to select "Language" from the "- Select a target -" dropdown - but there is no such option