Today I wake up with this simple idea. It also has to do with Jason’s
http://modxcms.com/forums/index.php/topic,1657.msg11465.html#msg11465
(i.e. versioning and multi-language content with the same stuff)
We can start from Susan’s implementation and add two (three) fields in the content table: refId and language (and version if we want to do page versioning). In this way, we have a "main" page id, that is the page id that we refer to, that is the page in the "default language" and then another page, with another id (but somewhat hidden to the user) but referring to the main page id, in which the content has been translated (and the field "language" contains something like it_IT, fr_FR, etc.)
We then need to add some code to redirect the browser to the correct id given the language from Susan’s cookie or session or what-you-want.
What do you think about this?
There is some work to do in the manager, because the pages with the same refId has to be somewhat hidden (i.e. do not appear in the document tree, but they should be visible while editing the "main" document) (I will do, if nobody wants to do it

)
Wendy, can you share just your idea, before it works, just to understand what you are doing?
And, finally, why do you think that XML is the right choice for the localization? (you are talking about snippets/modules translation, are you?) There has been one discussion in the past, for the choice of a translation db table, .po/gettext files and xml... and it never finished with a solution!