Yesterday I faced a problem that I had met before; I needed to change the master language of an EPiServer CMS site. I have done successfully using the awesome tool called Advanced Language Manipulation Tool for EPiServer by Adam Najmanowicz, but that time I used it on a EPiServer CMS 5 site.
It turns out it works like a charm in EPiServer CMS 6 as well, although the look and feel was a bit off:
However one thing that I forgot after I had updated the master language from English (en) to Australian English (en-au) was to enable the new language. The result was that I could not create any new pages, the option for creating new pages was disabled.
It took me a little while before I thought of enabling the language, but once that was done everything worked great! Just go to Language Settings for the Start page in Edit Mode and enable the language of your choice.