Fix and improve make update-fr-translation

[erikmd]

• Kind: bugfix

• Close Bug: file translations/fr.po.header is missing #555

Description

Fix and improve make update-fr-translation

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)

Note to maintainers

• Read this wiki page.
• Make sure the PR has a milestone.
• Assign yourself before merging.
• Either do a regular merge:
  • for PRs containing several commits following conventional-commits,
  • or for PRs containing 1 commit shared with a later PR (to preserve the SHA1)
• Or do a squash-merge:
  • for PRs containing only 1 commit (not shared with a later PR),
  • or for PRs containing several commits that need not be kept in the history;
  • → Update the commit message header with a conventional-commit type,
  • → Add a footer Close #… if a related issue exists.

[erikmd]

I believe this small bugfix PR can be merged without discussion, all the more as it is consistent with @yurug's remark in
#548 (comment)