Contao 3: how to customize Swift Mailer?

Hi.

Recently, Swift Mailer is updated to 4.2.1 on Contao 3 and I found that Swift Mailer has better support for E-mail in Japanese: a part of Swift Mailer document.

Now, what is the proper way to add these setting in Contao?

I don't want to add these setting unconditionally in Japanese page, but to apply conditionally by content of E-mail. I plan to create a simple extension to do this.

Best regards.