Going a bit nuts here. When I have products in my cart and click on Checkout, my shop switches back to main language. It only does this from the homepage!
My dev shop, with identical files and settings, does not do this. So it must be server side or something…
Anyone? Would really appreciate the help…
Thanks!
Just a guess…
Selected language is stored in a cookie.
When you go to the cart, you are changing the site name from http://xxx.com to https://xxx.com (or other url) (of course this assumes you have SSL setup for the checkout process). This probably makes the cookie invalid and it must be set again. I don't recall if they also store the language in the SESSION. If they do, then that would transfer with the protocol change to https.
Seems odd this would be first this would come up though…
- double
Thanks TB, turns out that it was a commercial add-on I had running on my homepage. Out of respect for the company I won't mention which one here, but if others have the same issue feel free to contact me. I'll contact the company behind the add-on too and inform them, i'm sure they'll fix it quickly.
Okay, assuming if it were one of mine, I would have heard something by now! 
Good you found it. I’m sure others would benefit by the debug process you used to discover it.