custom labels are not translated when locale is fr-FR

1. install Nuxeo
2. in Studio / Designer / UI / Translations
3. create a file messages-fr.json file
4. add custom labels like:

     "label.document.type.author": "Auteur",
     "label.document.type.book": "Livre"
   

5. create a document type using these labels
6. synch the server with the Studio project
7. set the browser language to fr-FR
8. create a custom document using the type newly created
9. observe the label is not translated
10. switch to fr language
11. observe the label is correctly translated
12. observe the label is present in the $NUXEO_HOME/nxserver/nuxeo.war/ui/i18n/messages-fr.json file

Note: this may be linked to previously fixed NXP-24817 issue.

Browsers tested: Chrome, Firefox

Expected result: all the labels should be translated being custom or standard