XML files hold data formatted by tags.
<example>
<data>
hello
</data>
</example>
You can name the tags anything you want, but some programs(like CSound) only read specific names.
Programming languages that have an XML parser can load XML files. You can program it to except any tags you want.
If you want to mix these languages and program together, you can make an XML parser in each language that uses the same tags CSound does, that way they will all be compatible with each other.