Because finding a clean, pre-made macos-sierra-10.12.6-french.iso is risky, the best practice is to build it yourself. You will need a working Mac with access to the Sierra installer.
A few important notes to help you:
For French-speaking users running legacy music production software (Pro Tools 10, Logic Pro 9), vintage scanners, or specialized industrial hardware, this ISO is a critical tool. macos-sierra-10.12.6-french.iso