manifest.json is a Chrome and Crosswalk manifest file. Specification is available here: https://developer.chrome.com/extensions/manifest and here: https://crosswalk-project.org/#documentation/manifest/using_the_manifest
You don't have to increase app's version number at manifest.json each time you've changed something, Crosswalk app with old version will be successfully reinstalled.

//LANGFIELDADDED label marks a place in code where you should add/change something in case you've added new multilingual field