Hi,
Sorry for the delay, it looks like your thread was validated in our ticket system by accident.
When HikaShop send the order emails, it uses the front-end language.
When an order is confirmed from the backend, HikaShop try to load the current language file or the customer to send the email in the right language.
But when the payment plugin confirm the order, if the language is not specified in the notification url ; the default language will be use. Because the payment notification is made in the front-end, the front-end language will be use ; the context is not the same than in the backend.
So you should check your payment plugin (I don't know if the problem is with iDeal, paypal or both), specially for the notification url and the "lang" parameter.
Regards,