The repack scripts currently handle system extensions that do localization via the chrome registry by replacing localization resources (.dtd/.properties files) and making appropriate edits to the extension's chrome.manifest. As we move away from chrome.manifest and toward the scheme defined in bug 1492593, the repack process will need to rewrite the affected fragment of system extensions' manifest.json files.

I think I misunderstood how repacks work. The chrome.manifest files aren't actually modified, they're just re-generated using substitutions on a variable called "AB_CD", eg: https://searchfox.org/mozilla-central/source/browser/extensions/formautofill/locales/jar.mn There's a bunch of magic there that I don't understand, but regardless I think the build system logic will need to be able to find the right .ftl files for a particular system extension/locale combo when we're producing a localized build.

With :kmoir's consent, I'm reviewing the backlog for bugs in FFx Build System to see if we can do some things to clean up the backlog. This bug appears to be an enhancement or feature request. If so, please set the bug's severity to `enhancement` and/or add the `feature` keyword depending on if this is an improvement to an existing thing, or a new feature/tool/system.

I don't really know what the guidelines are here for deciding what's an enhancement versus feature for the build system. This bug is about the build system keeping pace with ongoing work in localization of system addons.