I've been using this tip in order to add Facebook Messenger Chat Plugin on a website via Google Tag Manager, and it's working (many thanks!)
But when the visitor is not connected to his Facebook account, the welcome phrase is in English. The other translations managed through the Facebook app are working correctly when you are connected to Facebook.
I'm really not into code, but I changed this:
js.src = "https://connect.facebook.net/en_US/sdk.js";
js.src = "https://connect.facebook.net/fr_FR/sdk.js";
with no effect.