Prior to this change Field API fields created programmatically without specifying their translatability were non translatable, while fields created through the Field UI were translatable. This inconsistency has been fixed by making the Field UI default to non translatable.
Contextually the Field UI field creation behavior has been changed to allow to alter field translatability, which previously was hard-coded in the submit handler, through a
Other corrections to make core fields behave consistently with respect to translatability were:
- Make node bodies untranslatable by default
- Make node bodies untranslatable while upgrading from Drupal 6 to Drupal 7
- Make article images untranslatable by default