From the JS-side, there are calls like this to create string bundles, and they will never be invalidated when the app locale changes. We should be able to invalidate them through the nsStringBundleService, since it can listen to app locale changes.

XPCOMUtils.defineLazyGetter(this, "gBrowserBundle", () => {
  return Services.strings.createBundle(
    "chrome://browser/locale/browser.properties"
  );
});

Then this will work:

Services.locale.requestedLocales = ["en-US"]
console.log(gBrowserBundle.GetStringFromName("addonInstallFullScreenBlocked"))
// "Add-on installation is not allowed while in or before entering fullscreen mode."

Services.locale.requestedLocales = ["es-AR"]
console.log(gBrowserBundle.GetStringFromName("addonInstallFullScreenBlocked"))
"La instalación de complementos no está permitida mientras o antes de entrar a modo de pantalla completa."

Depends on D143745

I have some safety concerns with this and need to investigate more.

Comment on attachment 9272879 [details]
Bug 1764802 - Add a test for switching locales and string bundles; r?#platform-i18n-reviewers!

Revision D144032 was moved to bug 1761835. Setting attachment 9272879 [details] to obsolete.

I'm going to tentatively mark this as invalid since I'm not sure it's a good approach anymore. We instead migrate strings to Fluent.