I'm using i18n -- and it's all going very well -- except for a url tangle that happens with the front_page module and url aliases. I have a custom front page -- using "Full" in the front_page settings. So here is what happens:
1) You are on an English page and click Spanish to view it in Spanish. The URL changes from www.site.com/en/page to www.site.com/es/page. The "Home" link in the breadcrumb changes to "Principal." That's all fine.
2) If you click on "Principal" it takes you to www.site.com/es/frontpage, which looks just like www.site.com/en/frontpage because of the front_page module.
3) Then, when one clicks on one of the links on the English-reading front page, it brings them to a page with this url: www.site.com/en/es/page. As you could guess, that leads to a schizophrenic page -- the content is in English and the Navigation is in Spanish! Ack!
I installed the globalredirect module, which helps get things back on track (before I was occassionally and inexplicably getting non-aliased urls which would throw it all off in a different way), but it doesn't solve this problem.