List of all translations of a message in new interface
Access to translations in other languages page from new interface
I used to be able to access all the translations of a message listed on one page, via a link in the sidebar. I don't seem to be able to do this from the new interface, because I can't work out how to get from the new interface into the actual message sub-page 'message title/cy'. How do we do this?
If you mean that you want a link, there's no hope for that. Actually they want to progressively kill the appearance of message keys altogether. Your best bet is, as always, use Special:Translations and paste the message key in it; in the (rather near) future, we'll have Bug 46793 – Provide access to translations in other languages, so you'll only have to scroll a (fair) bit on the right side and then open the link.
We have removed the message key from the places where it should not be. We are not planning to do any further changes there.
Like said above, you should soon have a link to access that page. Though, it will be in the small down arrow next to message title in message editor, not in the sidebar as said above.