When I got to the admin section and edit the tab names, it makes my whole installation present a white page. (seems to be the standard 'something is wrong' functionality with sugar).

Anyway, i found that when i do this, it creates a file called custom/include/language/en_us.lang.php

But, it's an incomplete php file. I close the php tags inside that file and it brings the installation back to life.

Why doesn't this complete when i customize the interface? What could be wrong.