France Fichier des écritures comptables (FEC) required JournalCode and JournalLib to be populated.
it is mandatory fields and it should consistent.

Actual design has limitation as based on voucher setup.
Most of French customers / partners will create voucher with some code based but not all and this can bring file rejected by tax organisation

a. If voucher is only numeric, no Journalcode
b. if voucher is shared between journals, same Journalcode will have different JournalLib

Suggestion: to solve this issue, Belgium localisation Posting journal can be shared with France country company.
JournalCode will then be available on transactions.