Allow a WebformEmailHandler to specify the language of an email message.

This functionality appears to be functioning correctly based on my testing.
Here are the steps I followed:

• Created a simple webform with some elements and ensured they were translated.
• Added email handlers for the 'current', 'Dutch' (which is my current language), and 'English' versions.
• Submitted Webform.
• I observed that emails were sent appropriately, with the 'Dutch' and 'current' versions using the Dutch translation, while the 'English' version used the English translation. 😊

Thanks for the review. This was a very tricky request from a client. I am hesitant to merging it until a few people need it and use it.

Tested on a simple webform and works as expected. Thanks!

We'll test this in the following days on a client's complex conditional and multistep webform.