French translation fixes

[sunny]

Multiple fixes for the French translation:

• fix typos
• harmonize the usage of the informal tu rather than vous
• remove text that was left in english
• better wording
• translate text inside the code where it made sense
• prefer non-gendered turns of phrases when possible
• use French quotes («» instead of '')
• add non-breaking space before double-ponctuation characters (!?:;»)