On the page added in bug 1122256 we need to add in the ability to install a language pack by: - calling the appropriate API  - ensure that we don't record the install of the language pack - cope with the appropriate callbacks if they exist for the language pack install
For the record, it needs to be said somewhere: I thought I was clever by generating a minifest from scratch dynamically, without having to read the original manifest from the package, but this is bad idea. The platform will complain with MANIFEST_MISMATCH because it compares manifests (see https://dxr.mozilla.org/mozilla-central/source/dom/apps/Webapps.jsm?from=webapps.jsm#3791 )
API/Backend changes needed to fix comment 1 were merged in https://github.com/mozilla/zamboni/commit/aaeb2b9f3a8c449bb25ebbda9ea9e737c25a9c71
Fixed in https://github.com/mozilla/fireplace/commit/a0d06396d6be1aaec196fa18758d78934e5ef37f and https://github.com/mozilla/fireplace/commit/00e82d6aa55726d0e89ce91d7e397787845eb9d9 (merged in https://github.com/mozilla/fireplace/commit/21345851b7f64ff9bb2a057b14cdaa09e8d7c0ff) We re-use apps_buttons.js code to simply call installPackage() with the minifest URL.