Translation label custom field

Hello,

I have a problem with the translation of label for custom fields user (created with Administration » Configuration » People » Account settings).

On my register page, I added 2 fields:
- A textarea with a label and a description (where user adds some words about himself)
- A checkbox boolean with a label containing a link (for accept of the general conditions of the site).

English is the default language of the site.
I also installed the mondule i18n.

I tried to translate label of these 2 fields with interface translation :
Administration » Configuration » Regional and language » Translate interface (tab translate).
But this solution didn't work, label and description of the custom fields stay in English.

By looking on the net, I found a second solution which consists in exporting the French translation (a fr.po file):
Administration » Configuration » Regional and language » Translate interface (tab export)
Then I modify the custom fields in this file:
#: Field:field_champperso:user:label (label)
msgctxt "field_champperso:user:label"
or
#: field:field_champperso:user:description (description)
msgctxt "field_champperso:user:description"
By informing:
- Msgid for the original text in English
- And msgstr for the French translation
After do that for the label of my 2 custom fields and the description, I imported again the translation file fr.po in drupal :
Administration » Configuration » Regional and language » Translate interface (tab import)

When I return on my register page, the French translation takes place for the textarea: the label is translated as well as the description

However concerning the checkbox : nothing. The label stays desperately in English.
I try by removing the link contained in the label and the problem stays.

Maybe someone know a solution?