Switching language isn't a problem. It appears that there is an inherent flaw in the SEF module for JoomFish. From an SEO standpoint, duplicate content is a bad thing. JoomFish and your SEF module enable URLs extensions to be valid under every domain on the site. The URL should have a second requirement tied to a domain. Another example is now surfacing.
We do not use a filename for our "home page". We just use
www.domain.com/ or
www.domain.es/. If we use your SEF module for titles and meta tags we cannot duplicate the blank "SEF URL" because you have a unique index constraint on the "SEF URL" meaning a URL is only valid once.