The general rule of thumb is that a preview of a document will always be displayed in the language of the person viewing it. When a document is sent it will be in the language of the person receiving it. This ensures that the person previewing it can check for any errors, which they may not be able to do if they do not speak the recipients language.

There are a few exceptions though, when the document is intended to be printed out. These include:

Customer invoices. Once an invoice is issued it is always displayed in the cusotmer's language.
Delivery notes: Delivery notes are always displayed in the customer's language

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