I've merged the 2.2 dev branch into the 2.1 master branch, making 2.2 the current stable version. I've been switching my own sites between the two branches for weeks without issue, and I've not had any reports from others about compatibility issues, so believe it's safe to merge. However, there are a lot of changes between the two so use caution when pulling in the latest commit. By that I mean backup and test everything out to ensure all still works as expected. Of course, this is something you should always do, but especially so in this case (it merged more than 50 commits from the dev branch).
Language Support
The LanguageSupport modules are now included in the ProcessWire stable branch, but they are uninstalled by default. These LanguageSupport modules are still considered beta, as I think we need more people using and testing them before we can consider them non-beta. To install them, just click "install" on the LanguageSupport module in your Admin > Modules. It will install everything else. You can also uninstall the LanguageSupport modules just as easily as installing them by just uninstalling the LanguageSupport module (and it will uninstall the rest). I will be adding a Language Packs section to the Download page soon. Thanks to all those that have helped with making translations. In the coming weeks, we will also be making more and more of the core modules translatable, so we'll need continued help from our translators.
Special thanks to Avoine (http://avoine.fi) for being a sponsor of ProcessWire 2.2. You will see their name start to appear elsewhere on the site as a thanks for them helping to make the Language Support possible in ProcessWire.
Other changes
The LanguageSupport modules are the major drive of this version. However, there are several other changes and additions, including:
Add module dependencies support.
Add module auto install and uninstall support, to correspond with dependencies.
Add 'page-create' permission to Setup > Templates > Template > Access.
Update to latest version of TinyMCE (3.4.7)
Add new Page Clone module to core (may be installed from Admin > Modules)
Update error log file to include hostname. Made tab delimited rather than colon delimited.
Added built-in multi-site/multi-domain support optiona via config file /wire/config.index.php
Addition of several new hooks, especially in the $pages API var and files Inputfield.
Add new 'Site' link in default admin theme.
Numerous other bug fixes and optimizations, see the commit log for details.
Upgrading an existing installation
There is no formal upgrade process for going from ProcessWire 2.1 to 2.2, so all you need to do is pull in the latest commit (when you have time to adequately test it). If you aren't tracking the source on GitHub, then you'll want to download the latest ZIP and then do the following:
Replace your existing /wire/ directory with the new one in the ZIP.
Replace your existing /index.php with the new one in the ZIP.
Replace your existing /.htaccess file with the htaccess.txt file included in the ZIP.
That's all you need to do. However, I suggest renaming or backing up your existing /wire/, /index.php and /.htaccess files just in case you need to revert for any reason. This is standard procedure with any upgrade. Please let me know how everything works for you.
After installing, there is one change that may affect you if you are using multiple non-superuser roles with page edit access. A user must now have page-create permission on a Template in order to create a new page that uses that template. You'll see this setting on any template that is defining access, on its access tab. So if you have non-superuser roles with edit access, go in and add that page-create permission for any templates that they should be allowed to create new pages from.