Magento 2 is sending Emails in english if sent programmatically

I'm looking for hours now and I found some topics that are similar but none of them solved my issue.

I'm writing a Magento 2.3 Extension to import customers from a different Database. Everything works fine except for the email language.

I would expect Magento to use automatically the correct language (store config) when i'm sending the "New Account Email"

Here is the method I'm using to send the Email

$customer->sendNewAccountEmail();

If I send the "Password Reset" Email by clicking on the corresponding Button in the customer settings, Magento sends the Email in the correct language.

I tried different config settings and different ways of set the locale inside my import method. Obviously nothing worked.