One thing you can do is mix Unicode rtl/ltr overrides. If the default is RTL, use the LTR override to write in English, and use the RTL override to get back to normal.
‭, rtl is
For example, ahead I've done
first ‮ second ‭ third ‮ fourth in HTML and copy-pasted the result:
first second third fourth
If you copy-paste the line into some text area and play around with the arrow keys on your keyboard, you'll find it pretty easy to realise that "second" and "fourth" are indeed rtl.
So just give everyone a page that has these overrides in copyable format. Since these are zero-width characters, it may make more sense to have two buttons like:
<button onclick="prompt('Hit Ctrl-C and close','\u8237\u8237')">LTR</button>
<button onclick="prompt('Hit Ctrl-C and close','\u8238\u8238')">RTL</button>
on some webpage and let your users copy the characters from there. Note that the characters have zero width, so they won't see anything in the box.