Before we email hundreds of people or leave messages on their talk pages, we should show the translation administrator exactly what the message is going to look like (in particular because the message is constructed from both hand-written input and boilerplate text, which may result in unintended redundancies or contradictions).
Version: unspecified
Severity: enhancement
Do you also think of a "dry run" which shows who's going to be notified and where (or at least how many users, as the log does after the fact)?
Possibly confusing bugs which the translation administrator might be missing by not seeing the complete message are bug 39104, bug 39105.
Currently, https://meta.wikimedia.org/wiki/Special:NotifyTranslators does not contain a preview button at all, so I guess this title of this bug was meant to read "should *offer* preview". In any case, it seems like a good idea.
I also like Nemo's idea of showing the expected recipient number, similar to the log entry ("sent to x recipients"). Example: A notification that I meant to send to Arabic translators only just went out to every language instead. I'm pretty (but not 100%) sure that the "Which languages to notify" was filled out correctly, but in any case previewing the recipient number - or even just the list of languages to notify - would likely have prevented >1000 misdirected notifications from going out.