Currently, one cannot use translation notifications for translatable CN banners: They do not show up in the dropdown list of available translation tasks to notify translators about in https://meta.wikimedia.org/wiki/Special:NotifyTranslators . (Neither are they contained in the list of available translation tasks that greets willing translators at https://meta.wikimedia.org/wiki/Special:PageTranslation .)
If this should rather be categorized as a bug for the Translate or TranslationNotification extensions, please let me know. Bur for now I'm filing it as a bug for CentralNotice, because I think that it may have to do with the way CN stores the translation units belonging to a particular banner. E.g. these two pages contain the German translation for the banner "FDCpropreview20133_v1":
and they are collected in the translation group "Centralnotice-tgroup-FDCpropreview20133_v1". Perhaps it would be possible to organize them into a translatable *page* instead? E.g.
with the translation units called "text1" and "text2", so that the above two URLs would become:
(This might have other benefits too, e.g. that one could link to a wiki page showing the entire translated text for a banner just in that language.)
That's just me thinking aloud though. Any other solution that enables the use of TranslationNotifications for translatable banners would work too. For my current use case, I'm now resorting to setting up a separate translatable page and manually copying the translation text back and forth, but that doesn't scale.