How to get a custom field label in a specific language?
Turns out that using t(), doesn't work for translated field labels.

To get the translated field label text (in the current language), just use the i18n_field_translate_property function

  $field_instance = field_info_instance('node', 'field_application_area', 'products');
  $label = i18n_field_translate_property($field_instance, 'label');


Or, for a specific language :

  $label = i18n_field_translate_property($field_instance, 'label', 'fr');


Found on this related answer.