Yes, the ideawas to avoid mixing up a dozen languages in the same interface, which may end up being confusing. I expect partial translations to not be present in releases. If your first choice ever happens to be an incomplete one, the second one should fill in all the blanks.I wonder, isn't this rather something the locale kit should deal with (i.e. if the catalog is lacking a word, try the next language's catalog for it)? Isn't that something that must happen in case of local versions of languages (e.g. de_DE -> de) anyway? Or was it an explicit design decision to avoid showing more than two languages in the interface? If so, that certainly wouldn't help anyone who speaks a second language, but not English.
Thus, the behaviour may be changed in this aspect. Something involving catalog chains. And yes, it already works for local variants.