I've already Googled it, without concrete success. So, I'd like to know if someone knows a converter from XML to TeX. Something "simple" as
XML is a file format not a language of any sort so asking for a general XML to latex is like asking for ASCII to latex. In general it depends what is in the XML file, one would not expect the same latex formatting for an XSLT program as a XHTML document, just because they both happened to use an XML syntax.
In general you need to specify the styling you want for the XML language in use.
XSLT is perhaps the canonical tool for this, however it is possible to get TeX to read the XML directly and apply formatting from a stylesheet.
For example, one specific translation: If you want to translate mathematics encoded in XML as MathML to LaTeX markup you could use the pmml2tex XSLT translation available from http://code.google.com/p/web-xslt/source/browse/trunk/pmml2tex