Attach event listeners and handle those parts with JS. For internationalization, template on the server side, sending a different bundle for user agents in different countries. CSS is enough to fill the gaps with other internationalization concerns like rtl.

The platform provides enough to do forms. It's one of the few things the web is good at. It has been around for ages and works. I mean, what I'm using to type this is a plain HTML form. It doesn't need a massive JavaScript framework that heats up my CPU on every keystroke.