Update 8/21/14: Google today made the feature official by announcing it on the Inside Search blog. Head over to Search -> Settings -> Voice -> Languages to pick your languages.
Anyone who uses more than one language on a regular basis has probably felt some degree of annoyance with Google’s voice search system on Android. You have to dig way into the settings to change the language, and there’s no way to mix and match.
[Update: It’s Official] Google Is Rolling Out Support For Multiple Simultaneous Language Selections In Voice Search was written by the awesome team at Android Police.