Does anyone know the answer to the opposite question? How many articles on the English Wikipedia lack interwiki links? It is possible (but less likely) that the articles exist in both places, but haven't been linked with an interwiki yet. I find examples of that fairly regularly, but am not sure how common it is.
Carcharoth
I was just doing a sample of 50 pages on enWP anyway to estimate the mean number of interwiki links - it's around three, but clearly skewed by hitting very common topics. Anyway, it doesn't seem stupid to divide total articles in Wikipedias by 3 to get total unique topics: enWP probably has more unique topics than other languages. So, guess what, consistent with "one million to translate" as ballpark figure.
On the question posed by Carcharoth, it looks like around half of enWP's articles have no interwiki.
Charles