Based on the comments below a proposal in any language other that English can definitely have fully translated interface when it opens. Technically it is pretty simple to translate interface (especially with community help), however develop and moderate community in different culture can be really complex. Most probably the non-English proposal will be on hold after commitment reaches 100%, and after some time it launches. So it is just a question of time. Maybe it happens next month, maybe it happens next year.
However we are committed to support sites in other languages just as soon as we're confident that the site creation process is working fine.
But one thing I can report is that we will not create entirely-non-English site until the interface can be localized in their native language.
We're working on some localization projects (most recently, support for non-ASCII tags), but we're still a ways out from being able to support a fully non-English site.