I agree with Jeff. As I said before, I feel the StackExchange™ MultiCollider SuperDropdown™ is one of the features that best help beta sites grow. It has done wonders for us at Skeptics, and I would be surprised to hear we are alone. As such, I am against the concept of filters.
With that said, I think that, eventually, filtering by language will become a necessity. Right now, there's only one site that is not in English - GermanGerman - and it even allows questions written in English (really?). However, as the Stack Exchange Network expends into more and more language, the importance of this feature will increase. Discovering a new interesting or reading something thanks to the hot question is cool, but content in languages I do not speak are of no interest to me - or anyone, really.
It's not a priority right now, but it will be - eventually.