In a client's website, the sales emails is seemingly randomly (so far we've notice it for the order_new and the shipment_new) sent in another language that the language set for the store view the order was placed in.

I have put some logs in the email sending process to try to see at which point the storeId is changed, but so far the issue hasn't arise (or it did but my log don't reveal it).

What could, in your opinion, provoke Magento to use another storeId than the one store in the db?

Edit: Magento version 1.10.0.1, no email related custom development or 3rd party modules