Hi,
What happening when it's a guest checkout? Is it going out on the language they submitted the order?
Yes, when the order is created, the current language of the user is recorded with the order. That way, the system will send the emails in that language.
What about the admin emails?
The order notification emails sent to the admin are sent in the same language as the ones for the user.
If you want to test the result easily, you can edit the emails via the menu System>Emails and use the "preview" button at the top. Select the order for which you want to test and you'll directly see the result.
There is no option to change the way it works. It is technically possible with complex custom development but you'll have to hire a developer in order to change this.