Testing the features for live language switching has been pretty tough. It would be much better to have testing utils to make this easy. I've started working on these locally, but haven't been able to get them done in time for Firefox 100. This bug is for tracking following up with these utilities.

This would do something like:

const { LocaleTestUtils } = ChromeUtils.import(
  "resource://testing-common/LocaleTestUtils.jsm"
);

const langpack = LocaleTestUtils.createLangpack({
  locale: "es-ES",
  files: [
    {
      path: `browser/chrome/features/formautofill@mozilla.org/es-ES/locale/es-ES/formautofill.properties`,
      contents: `autofillHeader=Bueno`,
    },
  ],
});

Services.locale.requestedLocales = ["es-ES"]

I'm doing this in Bug 1764802.

I moved the patch over here to land it before Bug 1764802.

Backed out changeset 015c2c72a2a2 (Bug 1761835) for causing failures in browser_stringBundleInvalidation.js CLOSED TREE

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&searchStr=windows%2C10%2Cx64%2C2004%2Casan%2Cwebrender%2Copt%2Cmochitests%2Cwith%2Cfission%2Cenabled%2Ctest-windows10-64-2004-asan-qr%2Fopt-mochitest-browser-chrome-fis-e10s%2Cbc6&revision=015c2c72a2a2754c9d8b423fee8dc271d13512b7

Log: https://treeherder.mozilla.org/logviewer?job_id=375234749&repo=autoland&lineNumber=9257

Backout: https://hg.mozilla.org/integration/autoland/rev/cecd1565c462f9ca637a38dfbc0b2d6fc86236a5

This may still be worth completing, but I realized that you can reliably test locale changes with pseudo-locales if you remove the pref observers and always trigger intl:app-locale-changed.

I'm going to close this since I'm not going to get to addressing the review feedback, and intl:app-locale-changed should be good enough.