The Paragraph, Character and Object Styles each have ‘Export Tagging’ sub-menus where the styles can be mapped to HTML tags and CSS classes.
Is there any way to include HTML attributes in that mapping?
Example: <span lang=“fr”>Content here.</span>
If this is not possible out of the box with InDesign, is there an efficient workaround?
You can't add HTML attributes in InDesign. I don't know of a workaround except cracking the epub open and inserting them manually or with RegEx.