Oxid language issue - _sendOrderByEmail

I have some problem for receiving order confirmation e-mail for end user, shop language is English but the order mail subject only displayed in German language other mail content is displayed in English.

I changed the language as Dutch to English in Master settings->Core settings->Main

but same issue again occurred.
In finalizeOrder function up to $this->save() the order details getting lang as English ( [_iLanguage:protected] => 1), but after that set as German language ( [_iLanguage:protected] => 0).
how can i solve this problem.

when multiple languages are available, user receives emails in language he chose in frontend

yes, you are right, but i have configured only English in front end where as in mail subject alone displayed in German and other mail content are in English.