If the customer language is english and the users one is swedish.
At first you need Technical Features at User --> Access Rights
Go to Settings --> Translations --> Load a Translations, now select your "swedish" language to load.
Then you can select the language for users and partners and have the possibility to add translations eg "product Name" by click at the icon in the top right corner.
Now you will send a sales order from swedish interface by email to the englisch customer. The system do that automatically if you edit the email template before.
Go to settings --> technical --> email --> templates and edit/translate the sales order by email template. At template/advanced fill in the language field: ${object.partner_id.lang}