Not sure if there's a way to switch to french (prepopulated) but you can edit the text for all the emails that are sent out in the system settings.
Is that not set up on the ModX installation language?
depending of what language you use on the manager the email will be send on that language.
If not, you can find - Forgot Login Email - on settings, you will see the email template there, just change it.
Cheers!
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ysanmiguel.com