If you have "include_default_lang: false" set (so that your main language doesn't use the language code in the URL (example: site.com/ site.com/fr/ vs site.com/en/ site.com/fr/)) then the language switcher will switch to any non-default language you select, but is unable to switch back to the default one when selected.
If you have "include_default_lang: false" set (so that your main language doesn't use the language code in the URL (example: site.com/ site.com/fr/ vs site.com/en/ site.com/fr/)) then the language switcher will switch to any non-default language you select, but is unable to switch back to the default one when selected.