When someone mails me through the contact form I receive this text in my mailbox:
Op vr 22 nov. 2019 om 15:42 schreef … (Shopify) mailer@shopify.com
I want it in English and why does there stand mailer@shopify.com and not the customers email?
Hello @milyz
Vincent here from Shopify!
The language of the contact form and the notifications depends on the language chosen for your theme. So, if your theme is set to Dutch, the contact form and the notifications will be in Dutch as well, but if you change the theme language to English, then the contact form and the notifications will be in English as well. You can change the theme language by “editing the language” of your theme:
If you don’t want to change the theme language, then you can also edit the contact form manually by editing the theme code:
Because the contact form is sent via our servers, and not directly from the customer’s e-mail address, our servers are shown as the sender. However, when you reply to the message, the reply will go directly to the customer who sent the message.
I hope this clarifies the situation! 
Kind regards, Vincent