Adding I18N features to the MVC PHP LOGIN framework

Hello Panique and other contributors.
Because many updates and structural changes occurred in the main php-login repository since the beginning of 2014, It took me a while to redo the work I had prepared on the 4-full-mvc framework at the end of 2013 .
I recently took that time and re-forked the php-login repository into my php-login-lang one.
Since a few weeks, there were no changes in the main develop branch, so I could quietly set up all this stuff. 
So, here is the pull request for a full multi-language framework.

Basic features are : 
* provide a language selector form with languages available for the application. In the current pull request, English and French are fully functional, and German is provided without translations so that you can see how the app handles untranslated sentences. The form automatically detects the visitor's language and falls back to english when no available language fits. 
* provide a 'locale' folder with XML and DTD files to parameterize available languages, plus subfolders for each language . 
* provide a Lang class able to deal with these features, allowing translation everywhere in the application with a public static method Lang::__($key [,$argument]) 

Possibilities for this php-login-lang repository are multiple, according to the choice you will make (to accept pull requests into the main develop branch, or to reject them):
* my repository can live its own life and leave the main framework unchanged, but it wil need lots of refactoring when you will change the php-login-framework. My php-login-lang repository can be used "as is" by people who need full i18n implementation, but I won't be able to make time to handle several refactorings.
* this is why I propose this pull request, so that my repo is merged into the main one (and I will delete my repository as soon as it is done).

About the present pull request, feel free to tell me if anything is wrong and I will make the required changes. If you need, I can also send you by private message an URL where to test the php-login-lang live.

Thanks for your great work, 
Tristan