If we're talking about an HTML text input field, you can use `dir=auto`. Other tools should have similar features. This approach is also very common on paper, which naturally flows in the correct direction.

https://www.w3schools.com/tags/att_global_dir.asp

https://caniuse.com/mdn-html_global_attributes_dir

Edit: oh use case? Imagine a PDF form you'd like someone to fill out, or a page that may get printed. You could create two forms and let the person filling out the form pick their language, but the person processing the form may prefer to read the form in the other language.

> Your text inputs still need to flow either left-to-right or right-to-left though? In which use case would this be useful?

Anything that doesn't require input - particularly printed ones where you can't just swap with a button. Signs, menus, etc.

Layout on an airline magazine where the left side of the page is English and the right side is an RTL language like Hebrew or Arabic.