Currently there is no way to start discussion about translations with other translators. In translatewiki.net portal pages exist, but are rarely used.
In the new editor there is no link to message history, where you could see the last translator and use his talk page.
We discarded earlier the option to build threaded discussion inside the editor itself, but we could perhaps provide access to the talk page and indicator if there are comments on the talk page.
This doesn't seem to be often requested feature.