A Bit of Background
Stack Exchange was designed to make the Internet a better place to get expert answers to your questions. We don't want to restrict our software to English, but that's how the software was created initially; for an English audience — long before we expanded into a "network of global sites." It is our goal to make the Stack Exchange more of a planetary resource no matter what language they speak.
Our First Localized Site
The "German Language & Usage" site is a bit of a ground-breaking undertaking. The interface is still largely in English, but we don't want to tell our German users "sorry, you can't use our software." We are committed to support sites in other languages while we work through these issues and find ways to make this system work for the community.
We don't yet have a localize-able user interface and we don't necessarily speak your language. Crazier problems emerge when you start to deal with issues like bi-directional languages. But we are watching these first sites closely while we gear up to support non-English languages.
So when will this happen?
Without throwing out a lot of arbitrary, wildly-speculative dates, the best I can say is that localization has obviously become a larger priority. But there are no dates to report or even an estimate. There are more up-and-coming proposals targeted for other languages. We'll let those communities move forward, too, with the understanding that the menus and prompts will all be in English… for the time being.
A localized interface means that all the menus and prompts would be translated into other languages. Tags would have to support non-ASCII characters. We'll likely need to crowd-source the translation to the communities creating the sites. We need a way to follow, and understand non-English-speaking communities. We also need to come up with methods where we can learn and understand the cultural issues which will likely be virtually unknown to us. We cannot assume a site is running smoothly when we cannot follow (and understand) what is being discussed.
It's a big undertaking. Be patient and help us work through these transitional issues and work-arounds in the best way that keeps this community moving forward. We don't want to get mired down waiting for the software to perfectly support every conceivable world audience.