For iPhones, it's easy. Make sure you have WeChat versions 6.6.7, then just use the same scanning option you'd use for QR codes, then hit the new Translate icon down in the bottom right. It'll snap a picture and translate for you overlaid on the app, which you can then download.
Nifty. For now, only available between Chinese and English. Additional features in the latest update include "Voice Input," which is basically speech-to-text for English, Mandarin and Cantonese, and "Floating Window," which lets you swipe right on the SmartShanghai WeChat article you're reading which will minimize it into a little bubble that you can expand later. Nifty?
It'll probably roll out to Android in the near future, but in the meantime, you can use the Fanyi Baidu app (available on pretty much any app store available in China), which is actually pretty awesome. There's also Google Translate off the Google Play Store, which can work without VPN if you download the offline language package!