Javascript Strings and Localization

How can I localize/access Java Strings from background or content scripts?
My addon gets the following string from a .properties file via string bundle, then JSON.parse()'s it:

actionsjson    = [\
  {"code": "AudioFirstActive",\
                  "visual": "Audio Focus First Active",\
                  "tip": "Make first active Audio the current Tab (left)"},\

{…next entry…

where “visual” and “tip” can be localized.

You have to use:

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/i18n

This example is useful:

https://github.com/mdn/webextensions-examples/tree/master/context-menu-demo