This is a Python program to convert the proprietary flashcard list format used in version 4 of the Japanese app for iOS/Android into a CSV.
The .jpl format that the app exports is a gzipped json object containing a list of database ids for the entries. To decode this list you'll need a copy of Japanese4.db, which I can't post here on GitHub. You can get this file off of an Android phone from /data/data/com.renzo.japanese/databases/Japanese4.db after installing the app.