I write utility programs (converters, compression tools, etc.) with simple UI’s, which I put for sale of for free download on the Mac App Store.
I write these programs in English, and usually add a French localization (which I do myself). By comparing the distribution of sales per country between a localized app and nonlocalized one, I can estimate how much the French localization boosts my sales. (The apps themselves are pretty generic and not country-specific, which makes the comparison possible.) For example, on nonlocalized app A, my sales in France are 2.5% of all sales, while on localized app B, sales in France amount to 10%. This means localization increased my overall revenue by 8.4%.
My question is: does someone have an estimate of how much localization is worth for each language? (according to this metric, i.e. potential added revenue) This would be a great help to prioritize new localizations (which I cannot do myself).
 And not, that's not 8.5%.