There are two options to send bilingual emails:
You can send a single message with both English and French content. In this case you need to create an anchor link to jump to the part of the message where the other language begins.
If your English and French emails are two separate messages then you need to link to the French message from the English version and vice versa.
You can get the link to the online version of a message from the message editor:
How-to video to send bilingual or separate English and French emails: