i18n(app.yml): new language fr

[Gudsfile]

What does this PR do

Translated topgrade to fr.

Standards checklist

• [x] The PR title is descriptive
• [x] I have read CONTRIBUTING.md
• [ ] Optional: I have tested the code myself
• [ ] If this PR introduces new user-facing messages they are translated

For new steps

• [ ] Optional: Topgrade skips this step where needed
• [ ] Optional: The --dry-run option works with this step
• [ ] Optional: The --yes option works with this step if it is supported by the underlying command

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.

[SteveLauC]

Thanks for doing it!

[Gudsfile]

Thanks for the PR, found some tiny errors.

Oh! Sorry for the errors. Thank you for your vigilance and suggestions. It's been corrected.