Got it! I've read Beowulf and Canterbury Tales this way. It's helpful for comparing Middle English (Canterbury Tales) and modern English - as a native English speaker, I can understand the gist of Middle English, but it's super helpful to have the side by side version for all the different vocab and grammatical structures so you don't always need to be glancing at footnotes or endnotes. I try to just read the Middle English and glance at the translation as needed. As far as Old English (Beowulf), much, much fewer words are understandable to a native English speaker, so I read the modern translation, but it's still really cool to have access to the Old English to get a sense of the sounds and poetic rhythms.

As far as books where the writer uses multiple languages, the best example I can think of is Finnegans Wake. It's nominally written in English but contains a mishmash of other languages to create multilingual wordplay. I've heard you need to understand 4 or 5 languages to really understand it without copious footnotes. Also, a lot of non-French classic novels have a fair amount of French in them because at the time most educated Americans, British, and other Europeans knew it. For instance, War and Peace has a lot of French dialogue.