1.7.3 is introducing quite a few improvements concerning multilanguage and languages in general.

A. Multilanguage

1. In 1.7.1 was introduced the new Multilanguage Status Module: https://community.joomla.org/blogs/community/1501-171-new-multilanguage-status-module.html
In 1.7.3, a necessary improvement is implemented: checking the status of the Contacts linked to an Author, as it is necessary to create a unique contact per author and tagged to 'All' languages OR one contact per published Content Language.
If an error occurs, a message is added in the modal with the user name concerned (Thanks Marc Studer and Christophe Demko):


2. It is now possible to customize the Site Name depending on the Content Language displayed.
A new parameter has been created in Extensions=>Language Manager=>Content tab=>Edit a Content Language:

3. By enabling a parameter when editing the Language filter plugin, users will at last get a new feature that has been expected for long:
the Content Language prefix will not be used anymore in the url for the Site default language when SEF is on!
(Thanks to all who participated in trying to patch this —See: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=25037 — and to Christophe Demko who solved it.)
Extensions=>Plugin Manager=>Filter by System=>Edit System - Language Filter

Front-end result:

4. The batch processing in various Managers in back-end now includes a Language field.
It is now possible to batch-change language for ticked items, as well as batch-copy including Language change to another Menu (in Menu Manager) or another Category (Thanks Michael Babker). We hope to also include Menu Associations in this process for 2.5 .

B. General Languages improvements

1. Global Configuration now includes a default Offline Message per Language pack.
If this option is chosen, the offline message will be customized depending on the site language displayed.
It is always possible to override this value by using an xx-XX.override.ini ( See https://docs.joomla.org/International_Enhancements_for_Version_1.6#Language_String_Overrides )

2. The Administrator Help menu may now propose different forums to link to, depending on Language.
A registered language pack may propose a link to the Official Language sub-forum on forum.joomla.org. Custom distributions may propose links to their own external Community Forum by adding in their customized sample data the said link.
Users may decide of their preferred help forum.
Module Manager=>Filter by Administrator=>Edit Admin Menu

3. A new column has been added in the Installed Languages Managers to display the Language tag (en-GB, fr-FR, it-IT, etc.)

Thanks again to all who participated in implementing these improvements and do not forget to upgrade to 1.7.3 !

And last but not least, we do now have an official multilanguage demo site (now updated to 1.7.3) where users can log in back-end to make sure they structure their site correctly:
See: http://multilingual-joomla-demo.cloudaccess.net/