Configuring modules for email sending and formatting is a pain and confusing. Most mailing modules rely on Mailsystem module to help configure this.
In D7 Mailsystem 3.x there's an improved method of doing this by using a delegator class. https://www.drupal.org/project/mailsystem/issues/1508104. I propose that the class and related functions be put into core. The UI can stay out of core.
This will help any combo of modules share the responsibility of sending emails. One module can be used for formatting HTML (MimeMail, HTMLmail) and another for sending (SMTP, PHPMailer, Postmark, SendGrid, ...). The popular modules can then use the new class to allow other modules to share in the work and also provide a simple UI (without needing to install Mailsystem if they don't need that extra power).
Recent comments
I have re-installed from a clean minimal profile. When adding new translatable content, if it also has a menu item, the links are not synchronized and needs to be added manually...
xmlsitemap module generates wrong links for languages
This is not the case for me. I have started over a new installation with minimal profile and will enable things one by one. Will let you know how it goes.
xmlsitemap module generates wrong links for languages
The first problem is not a problem, things work as designed. When you install Backdrop in English, the existing content does not have a selected language. Then, when you add other languages...
xmlsitemap module generates wrong links for languages