Finally something I can comment on a bit. You may not be aware that the term "formal language" will have very specific denotations for a computer scientist (where it means
something that has a grammar associated with it like a computer language), which I don't think what you had in mind, since you mentioned XML.
XML is only formal to the extent that it has a Schema or a DTD associated with it. To be honest, I heard a lot about DTDs 10 years ago when XMLs were a newer topic, but less so recently, when lighter-weight specifications like
YAML or
JSON have dominated.
I'm a big Json fan. There are parsers in most programming languages. I use python for most of my work, and json specifications are almost equivalent to python dictionaries.
Sorry for the long digression. Here's the point. Language specifications, parsers, xmls, etc., all distract you from what you want, which is a description of whatever it is you want to
exchange or
archive. If you can tell me what information is important to you, I might be able to give you an idea how to make a simple json for it.