Google announced it can now understand more than one language spoken in the same speaking session. Google said, "with the Google Search app on Android, you can speak in multiple languages and Google will understand you no matter which one you choose."
Now you can switch back and forth easily with languages, as long as you stick with one language within the same sentence, and Google will understand it. You can select up to five languages total, which should cover the average person's language capabilities.
You can try it by going to settings in the Google App and clicking on Search & Now -> Voice -> Languages, and select your languages.
Forum discussion at WebmasterWorld.