The difficulty here is to determine which words to hyperlink. I once did something like this by excluding words from a list of stopwords (15.000 most common german words, drew an arbitrary line there):
It worked to exclude latin medical terms, but probably won’t work for many other use cases.
If you got a specific online dictionary in mind, I could provide a semi-automatic solution using my add-on Tippy Tooltips, if you’re interested. Here’s an example for Wikipedia links: Tippy Tooltips [Official Support] - #31 by kleinerpirat