Say I want to use some relatively common phrase in my addon, like jetpack ferret pizza lawyer. But of course, other addon authors have also used that phrase, and so it's in the wowace translation system for another addon.
Does the translation system merge this stuff behind the scenes? (In this case, my addon's translation pages would automagically pick up the other phrases that have gone before. That would be wonderful and efficient and other positive adjectives involving bacon.)
Or do duplicate phrases sit in isolation? If this is the case, is it possible to search for phrases across addons and copy in their translations?
Unfortunately, they are not shared. Equally unfortunate is the inability to search for localized phrases across the spectrum. However, the entire system has been recently exposed to actual maintenance so filing a CurseForge ticket on the subject would be a helpful start - as would PMing myself and whomever else in this thread who expresses an interest in this concept with a link to said ticket so that we may vote it up.
Turns out ticket 1478 already suggests this exact thing (but has been sitting in the usual "voted up, going nowhere" status for 27 months and counting). It's really good to hear that somebody is maintaining it now!
Given that the meaning, and hence the translation, often depends on the context, I am not sure that systemically and automatically sharing translations would help. However, some site-wide search (or import) feature could be useful.