Using a MutationObserver to reformat numbers in the user's locale
One issue I'm seeing is that you're listening to the entire tree. While not an issue in small apps, it might be an issue for huge apps, with deep trees. Consider limiting it to the places where you expect your custom tag will exist.

If you are in control of the process of adding these elements to the DOM, consider doing this transformation there instead of listening to the entire DOM for you to change it. It will be easier to debug since the transformation is involved in the process of generating that element, instead of an indirect side-effect of some other operation.

You know that feeling when someone else's JavaScript re-arranges the HTML you rendered on the backend and you had to hunt it down? Same story.