The selected language designated as "primary" should be the language that appears in your menus and controls, no matter what the secondary languages are. You make the language "primary" by moving it to the top of the Language and Region side pane in the selection box. Then you restart your computer. So, your primary language should be listed in first position, and French should be in second position. When you drag a language to the top, and restart your computer, it will become the primary language. If the above procedure is not working on your Mac's iMovie, you might try deleting preferences.
To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If still no luck, and your region is set to Europe, you might try changing it to United States. You can always change it back. See if that makes a difference.
-- Rich