How to change Captivate interface language?

I'm not sure to understand your question correctly: if you want to change the interface of CP itself, you have indeed to reinstall as explained in another reply. But since you mention 'translating' I believe that your question is more about the language used in the created text captions, the buttons, during the recording of a project. Could I be right?

You can change the language for captions, buttons etc while creating the project, going to the advanced button of the preferences. Or if you are going to record slides, in the small menu, under the button (camera) where you can choose the panning, audio and recording mode, you click on the Advanced button and can choose the language for captions. You need to have the right RDL-file in the Captivate directory for the language. This file can be edited if you want to change some captions.

Default labels for Quizes can be changed in CP4 itself in the menu Quiz, Preferences.

If you want to use same starting files for trainings in different languages, I can recommend the File, Export, Project Captions... where you can translate all captions and import them to have all captions in another language. Of course, be sure to control the result, but it works pretty well to my experience.

Hope to have been of some help; Lilybiri

Not sure if this will help but it's worth a shot...  By any chance is your systems default language French?  I believe the install automatically defaults to your systems default language.  If this is the case, try uninstalling it, change your systems language to English, then re-install.

Again I've no clue if this will work, but it's something to try!.