Cart always goes back to default language

My default language is English, and I am translating my site in French (fr_FR).

When I’m on a cart page in the checkout process, any sort of ajax call or button click (i.e. “Checkout”) brings back the English content – not the French like it should.

Is there an existing fix or this, or at least a workaround? I’ve modified the plugin too much to download a fresh copy now, so hopefully I can just add some minor fixes if necessary.

https://wordpress.org/plugins/thecartpress/