How to localize Firefox SDK Add-on name and description?
There is currently no way to do this from package.json. Read bug 661083 for more details.
However, there is a workaround: Manually edit install.rdf to add em:localized properties.
To do this you'll need to use the SDK to package your app into an xpi file. Then open the xpi (it is a zip file) and you'll see install.rdf in the root of the directory.
The MDN article Localizing extension descriptions describes what structure the em:localized properties needs to have.
This functionality has been added to the jpm tool recently (February 2016, see issue 495). Make sure that you use a recent jpm version, then the following code in package.json will work:
"title": "Default add-on name","description": "Default add-on description","locales": { "de": { "title": "Add-on name in German", "description": "Add-on description in German" }, "ru": { "title": "Add-on name in Russian", "description": "Add-on description in Russian" },}