Interface translation of custom fields
Here is Internationalization pack for building multilingual sites in Drupal: i18n.
To translate field labels you need “Field translation” module from this pack. After you enable it new tab “Translate” appears on field edit page.  



Update

That's strange that you can see your field labels on admin/config/regional/translate/translate page, so I started an issue #1672694. I think it's a bug.

Update 2

Fixed since Drupal 7.22