I changed my mind since the two functions wp_head() and pll_the_languages() can have two different links in case someone chooses to force the language switcher to link to the homepage.
So I created a new filter 'pll_translation_url' which you use the same way (but is fired sooner in the get_translation_url function).
You can try it with the latest development version (1.1.1.7)
http://downloads.wordpress.org/plugin/polylang.zip