For some flash cards for my daughter, I put English on the front and Japanese on the back (up-side-down), so that I can show her the English word and flip the card quickly and flash the Japanese, too. I think doing several languages at once is important. At one point in time they need to be shown that one word in one language equals another word in another language... and realize that both words have the SAME meaning.

I also made home-made books (with words on one page and the corresponding pictures on the next page), and just replaced the word pages with the same words but in a different language every now and then. In my case I made English, Japanese, and Spanish word pages.