It is because, you might be having multiple languages in your site but have not created the email template for all the languages. For example: You are having following such languages in your site (English, German, French) and created the email templates only for the language English. If customer places an order using the language French, then he will receive the default email because there is no email template for the language French.

So always you should make sure that you have created email template for all your site's languages.

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