Modification of HTML mail template I am using HTML Mail module. I have created an html email template specific to my form(webform) in the template folder of my theme directory. My form name is newsletter and the template name is htmlmail--webform--newsletter_email.html.twig

I want to override the email body based on a particular language. But my problem is that hook_mail_alter doesn't work in my case.

Drupal API documentation for  hook_mail_alter says that hook_mail_alter() allows modification of email messages created and sent with MailManagerInterface-&gt;mail().

Any help?
The hook hook_mail_alter() should work in this case.

Have you double checked if your module is activated and you have cleared your cache after adding the hook?

Also i am not sure if your template will work, because normally all templates are working with dashes instead of underscores.