[Django]-Why Django translation does not use the base language in javascript?

Did you try using rosetta? If you use rosetta then it will make your translations much easier.

we encountered exactly the same problem,
an easy way to fix this was to launch the following command after your django-admin makemessages -d djangojs -l fr

msgen PATH_TO_YOUR_DJANGOJS.PO_FILE -o PATH_OF_YOUR_DESIRED_OUTPUT_FILE

msgen will fill the empty strings with original msgid text, and is an internal tool installed with gettext utility.

Br.