Thanks very much - that worked just fine.
To be clear:
- I set the starting URL as [ External links are visible to forum administrators only ]
- I emptied the "Exclude URLs", "Do not parse URLs", "Include ONLY URLs" and "Parse ONLY URLs" fields
I had to adjust my .htaccess file, since URLs such as /en on my site are virtual - I have a central URL handler, so for SEO purposes I had stripped the final '/'.
I adjusted the .htaccess file to allow for 'mysite.com/en/' (and force the trailing '/'), while removing the trailing '/' from lower URLs (mysite.com/en/about_us).
Extract from .htaccess shown below. YMMV.
# force trailing '/' on bare language specifier URL (eg mysite.com/en/)
RewriteCond %{REQUEST_METHOD} GET
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?([^/]{2})$ $1/ [R=301,L]
# remove trailing slash on lower-level URLs (eg mysite.com/en/about_us)
RewriteCond %{REQUEST_METHOD} GET
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?([^/]{2}/.+)/$ $1 [R=301,L]