Google has officially made it possible to use the voice search with different languages simultaneously. It means you can add up to 5 languages and Google Voice Search application (read it Google Now) recognizes the language as you speak.
To add your favorite languages for the multiple language recognition feature do as follows:
1. Open Google application or simply open Google Now (Make sure the latest Google app version is installed)
2. Tap the 3dot icon and go to Settings
3. Under "Search & Now Cards", tap the Voice option
4. Go to Languages to add multiple languages
5. Select the languages you'd like Google to recognize by marking them. To select a language as a primary language, you need to touch & hold on that very language.
6. From now on, Google automatically distinguishes the language you are speaking and gets back to you with the proper search result.
** Note that the Google Now hot-word, OK Google, still works in English.