When I translate field labels via admin/config/regional/translate/translate then my changes are happening in the frontend - but this does not work for fields which I created myself. Those fields are available for translation in the same spot but changes to them have no effect - it always uses the original text.
In my case it's about some additional fields in the registration form. I have de/user/register and fr/user/register and the pre-existing fields are in the correct language and if I do changes to them, then they are applied. But if I provide translations for the fields which I added, then they just don't show up.
I am using Drupal 7.14. On the picture, you see the custom field "wohnort". The first one has been there right after the creation of the custom field; the second one only after the Field Translation module and the i18n pack.