Adding documentation relating to the Language files.

I have added a new section and index page related to the Language files used to translate the Umbraco UI as they are not currently documented. This will enable us to also update the invalid URL in all the language files so that it points to a valid documentation URL.

This is related to my PULL request on the core product:
https://github.com/umbraco/Umbraco-CMS/pull/546