I accidentally discovered it while testing
MDL-37477, I noticed that language menu is displayed on admin pages in Clean theme but not displayed in Base. Digging into the code I noticed that base looks at $PAGE->layout_options['langmenu'] before displaying language menu and Bootstrapbase, Clean and More don't.
config file for bootstrapbase defines 'options' => array('langmenu'=>true), for some layouts just as well as in base. But they are ignored when displaying the page.
Interesting that 'nonavbar' layout option is respected in CSS only. So for example, on frontpage the navbar is still rendered but hidden by the rule:
I remember this obsession with CSS caused