The core localization seem pretty stable but unfortunately we have a big problem now because when you upgrade to DNN 712 the language parameter is gone so al the pages must have a unique translation.

The localization in a clean install of DNN 712 works great!

 Thanks for your help again!

If you are having a problem when turning on Advanced URLs for a previously localized website, be sure to read Content Localization Walkthrough. The key statement is:

The way language specific site aliases work is a break from DNN versions prior to 7.1. Since DNN does not automatically upgrade from human friendly URLs to advanced URLs, it is also not able to automatically add language specific aliases. That means that after an upgrade from a pre-DNN 7.1 version, and a switch to advanced URLs, you will also need to add the language specific URLs.

I was having problems specifically with pages that had the same name in both EN and FR (e.g. Resources or Contact). When I was viewing the English site, selecting Resources would call up the French page, for example. I added language-specific site aliases to my website as documented by the above article and everything worked again.