You can decompile an apk on Android device using this : https://play.google.com/store/apps/details?id=com.njlabs.showjava
For more info look here: http://forum.xda-developers.com/showthread.php?t=2601315
EDIT: 28-02-2015
For decompiling an apk you can use this tool: https://apkstudio.codeplex.com/license
If that doesnt help check this link