Since years we format numbers, dates, and prices for all kind of locales in our backend. It is even part of most standard libraries.
But how about your frontend? Does it know how to format the numbers for every locale? And why is this such a pain in our frontends? It should be easy!
I will point out the problems that you have to deal with integrating l10n into your frontend and will tell you how to tackle them with the help of libraries that we already use.