'simple' isn't a valid language code, though we're outputting it for interlanguage links.
We 'could' add in a simple hack here that will make 'simple' output lang="en" instead.
Though I do have a bit of a more interesting idea. Instead of what, how about we swap simple for en-x-Simple and add in a code that lets us create aliases for language codes so that simple: will still be equivalent to en-x-Simple.
Going by bcp47 (https://www.rfc-editor.org/rfc/bcp/bcp47.txt) the code en-x-Simple is valid. It's an 'en' lang code with a private subtag of 'Simple'. bcp47 reserves x-* for private use purposes, things that wouldn't be registered, essentially that's what we're talking about here.