...It is NOT intended for piracy and other non-legal uses. It could be used for localizing, adding some features or support for custom platforms, analyzing applications and much more. The decode option on Apktool can be invoked either from d or decode. In order to run a rebuilt application. You must resign the application. Android documentation can help with this. As you probably know, Android apps utilize code and resources that are found on the Android OS itself. These are known as framework resources and Apktool relies on these to properly decode and build apks.
APKRepatcher helps you to modify an existing apk with simple GUI
...Additionally, it provides you option to convert Dex, Jar, Class, Smali, Class from one format to another. APKRepatcher makes use of dex2jar, jadx, rsyantaxtextarea, zip4j, apktool