  The use of programming languages such as Java and C in Open Source Software
(OSS) has been well studied. However, many other popular languages such as XSL
or XML have received minor attention. In this paper, we discuss some trends in
OSS development that we observed when considering multiple programming language
evolution of OSS. Based on the revision data of 22 OSS projects, we tracked the
evolution of language usage and other artefacts such as documentation files,
binaries and graphics files. In these systems several different languages and
artefact types including C/C++, Java, XML, XSL, Makefile, Groovy, HTML, Shell
scripts, CSS, Graphics files, JavaScript, JSP, Ruby, Phyton, XQuery,
OpenDocument files, PHP, etc. have been used. We found that the amount of code
written in different languages differs substantially. Some of our findings can
be summarized as follows: (1) JavaScript and CSS files most often co-evolve
with XSL; (2) Most Java developers but only every second C/C++ developer work
with XML; (3) and more generally, we observed a significant increase of usage
of XML and XSL during recent years and found that Java or C are hardly ever the
only language used by a developer. In fact, a developer works with more than 5
different artefact types (or 4 different languages) in a project on average.
