Nice job! There have been quite a few of these language substitution extensions over the years. (Language Immersion, Polyglot, MindTheWord, etc.)

I have a personal extension that I wrote (close to 12 years ago at this point) which does the same thing - translates random words on websites as you browse according to your linguistic level. It vastly predates LLMs though so it's all built on sentence segmentation, POS analysis for stemming, and other NLP techniques.

I've written a bunch of integrations for it so it works with websites, documents, even Kindle books.

https://mordenstar.com/projects/linguaswap

Now onto some feedback:

The site is visually a bit of a mess. The nav bar anchors but not to the top of the viewport (scroll and watch). Some of the cards are also different sizes. Some of the text isn't properly spaced (look for the colons).