I am currently working on two major XSLT 1.0 projects.

* A cross-platform templating XHTML engine which is entirely based upon
   XSLT, for The Atom Syndication Format.

* A document uniformication framework, which has additional
   completmentary API to provide critical features of XPath 3.0.

Both projects are currently executed with Python module Python-LXML.

What computer language is required to work with Xalan, in case I want
to upgrade these projects to XSLT 3.0?

Apache Xalan Java has an XSLT 3.0 development branch, see
https://github.com/apache/xalan-java/tree/xalan-j_xslt3.0_mvn. So it is
Java.

If you work with Python and want to try XSLT 3.0, you could consider
SaxonC and its Python wheels, e.g. SaxonC HE at
https://pypi.org/project/saxonche/.