Hi Melanie,
The 17.0.1 version of MapInfo Pro is released which is a multi-language installer that installs English, German, French and Japanese versions of the software. The language of MIPro by default is the system language taken.
Coming to your question, there are 2 ways of doing it. The recommended way is to uninstall your English 17.0 MIPRO and run the 17.0.1 installer. While uninstalling, do not return the license. If you have the French OS/locale it will automatically install French version for you. The serial number will not have to be changed.
You can find more details in the MapInfo Pro 17.0.1 Release Notes.