I have worked pretty a lot analyzing Android projects with different reverse engineering tools, generally for secure purposes, to see what can be seen by a hacker when he/she gets my app. I've met a problem that I couldn't find any of decompilers capable to decompile files of "XAPK" format (not simple APK). I know, asking for a way to find a tool is not kind of questions good for stack overflow, but I am pretty sure there are many people with the same problem and I find it highly relevant. I tried apk-tool, decompilers provided with androguard tool, online-decompilers, none of them are capable of working with XAPKs.
The .xapk is just a ZIP file which contains the original .apk along with some other files. You can simply unzip the .xapk and decompile the .apk within.
For more details, there's Decompiling XAPK Files.