I heard it is Python
ubuntu-desktop is a meta packages that only has one purpose, which is to install a collection of packages by the Debian package system. Therefore, the package itself has no source code in this sense, but only information for the package system.
What you probably really mean is about the packages that are installed when you install
ubuntu-desktop. However, this is a very broad question, since it is a collection of packages.
Some packages are written in C++ (as the main Gnome windows libraries are). Other packages use the Python gtk bindings and are hence written in Python. There are also packages that are written in mono.
I do not think, that there is a list that is kept keeping count of how many applications are written in which language. This would also be difficult to keep maintained, since the set of packages being "part of"
ubuntu-desktop vary from release to release.
Poked around in Launchpad: ubuntu-desktop to and browsed the source for a few mins. It appears to be a mix of Python and shell scripts.
I think the question referred to the language used to write the applications running on the default installation.
It's hard to say which language is used the most, but i would guess C or C++. This is just a guess and since all languages are pretty equal in terms of outcome, it doesn't really matter.