intl.accept_languages changes when the user changes their OS language

When the OS language changes, FF mobile updates the intl.locale.os and intl.accept_languages preferences.

      case "Locale:OS": {
        // We know the system locale. We use this for generating Accept-Language headers.
        let languageTag = data.languageTag;
        console.log("Locale:OS: " + languageTag);
        let currentOSLocale = this.getOSLocalePref();
        if (currentOSLocale == languageTag) {
          break;
        }

        console.log("New OS locale.");

        // Ensure that this choice is immediately persisted, because
        // Gecko won't be told again if it forgets.
        Services.prefs.setCharPref("intl.locale.os", languageTag);
        Services.prefs.savePrefFile(null);

        let appLocale = this.getUALocalePref();

        this.computeAcceptLanguages(languageTag, appLocale);

        // Rebuild strings, in case we're mirroring OS locale.
        Strings.flush();
        break;
      }

the method computeAcceptLanguages is the one responsible to keep the intl.accept_languages up to date.

  computeAcceptLanguages(osLocale, appLocale) {
    let defaultBranch = Services.prefs.getDefaultBranch(null);
    let defaultAccept = defaultBranch.getComplexValue("intl.accept_languages", Ci.nsIPrefLocalizedString).data;
    console.log("Default intl.accept_languages = " + defaultAccept);

    // A guard for potential breakage. Bug 438031.
    // This should not be necessary, because we're reading from the default branch,
    // but better safe than sorry.
    if (defaultAccept && defaultAccept.startsWith("chrome://")) {
      defaultAccept = null;
    } else {
      // Ensure lowercase everywhere so we can compare.
      defaultAccept = defaultAccept.toLowerCase();
    }

    if (appLocale) {
      appLocale = appLocale.toLowerCase();
    }

    if (osLocale) {
      osLocale = osLocale.toLowerCase();
    }

    // Eliminate values if they're present in the default.
    let chosen;
    if (defaultAccept) {
      // intl.accept_languages is a comma-separated list, with no q-value params. Those
      // are added when the header is generated.
      chosen = defaultAccept.split(",")
                            .map((x) => x.trim())
                            .filter((x) => (x != appLocale && x != osLocale));
    } else {
      chosen = [];
    }

    if (osLocale) {
      chosen.unshift(osLocale);
    }

    if (appLocale && appLocale != osLocale) {
      chosen.unshift(appLocale);
    }

    let result = chosen.join(",");
    console.log("Setting intl.accept_languages to " + result);
    this.setLocalizedPref("intl.accept_languages", result);
  },