hdiutil detach /Volumes/Install\ macOS\ Sierra hdiutil convert /tmp/SierraFrench.dmg -format UDTO -o ~/Desktop/macos-sierra-10.12.6-french.iso mv ~/Desktop/macos-sierra-10.12.6-french.iso.cdr ~/Desktop/macos-sierra-10.12.6-french.iso
Once you have Install macOS Sierra.app (French version), convert it to a bootable ISO with these Terminal commands: macos-sierra-10.12.6-french.iso
host pre-made ISOs, though official sources are always recommended for security. exact Terminal command for creating a bootable USB instead of an ISO? macOS Sierra (10.12.6) Download - Apple Support Community macos-sierra-10.12.6-french.iso