When you're dealing with bilingual content you have two options:
Sending a single message with both English and French content is the simpler option. In this case you simply create an anchor link to jump to the part of the message where the other language begins.
If your English and French messages are separate messages then you need to link to the online version of the messages. Link to the French message from the English version and vice versa. There is no built in toggle between the two versions but there is a workaround:
Get the link to the online version of the message from the list of messages page, remove the IDs after the last / and use that link in your messages.
For example if the link of the online version is
after you removed the IDs following the last /