unique identifier for the same pages in a multilanguage/site context

For people, as me, that comes to this thread, looking for a solution, pll_get_post() is your function.
I think this is the easiest solution for this problem.

    $the_translated_page = get_page_by_title( 'Custom page' );
    $the_translated_page = pll_get_post( $the_translated_page->ID );
    $the_translated_page_URL = get_permalink( $the_translated_page );

I’m pretty sure that this function (pll_get_post) was recently included in the plugin, or at least, it didn’t exists in 2013…

Hope it helps!