OmniFocus will use whatever language your Mac or iOS device is set to use. On the Mac, you can download the
English-only version if you'd like to force the app out of French.
On an iPhone/iPod/iPad, though, I think the only thing you can do is to set your entire device to run in English from the settings app.
If you have questions, we'd love to help! Please feel free to send them to our
support ninjas in French - we have someone on staff that speaks and reads it very well.