I understand how valuable built-in multi-language support would be, and I hope this can become a default feature in HubSpot soon.
Howeve, unfortunately, it looks like the beta is no longer available at this time.
However, if you have Marketing Hub Professional+ or Content Hub Professional+, using [custom templates](Use system page templates) might be an option for your needs.
Since the language slug is included in the URL of the subscription preference page, it may be possible to fetch the language from [the URL](https://stackoverflow.com/questions/1034621/get-the-current-url-with-javascript) or [the browser settings](https://stackoverflow.com/questions/1034621/get-the-current-url-with-javascript) and then update the content dynamically using [JavaScript](Modify DOM Elements with JavaScript).
Hope this helps!