little fix for setup.php to save during installation chosen language

[dniesel]

I made a few installations of GetSimple in the past few days. After the third installation I realised, that it wasn't my fault that the language which I chose during installation, wasn't saved to website.xml. I stepped into it and found out, that on install.php you can choose a language (if you have more than en_US in your language folder on setup), but on setup.php this parameter isn't handled. So GetSimple falls back to en_US because the global $LANG is empty at this moment.
So I directly include the chosen language (that for already the second step on installation will continue in your selected language) and write it to website.xml

Attached patch in based on r116