For example, let's say my document has two versions, in English and Italian. Would it be possible to code it so that pandoc could select one and parse the document to one language at a time?
There's no built-in way to do this, but there are a lot of ways to do it with additional tools. For example, you might try using a preprocessor: https://adityam.github.io/context-blog/post/markdown-with-gpp/
You could also use
<div lang="italian"> and
<span lang="italian">, together with a custom filter. See http://johnmacfarlane.net/pandoc/scripting.html.