Bug 42349: Change the RFPTarget in the region spoofing.

One of our patches makes sure that when spoof English is not in use, we
report the same locale as the app language, instead of reporting the OS
locale (which is Firefox's default behavior when the app and OS
languages are the same).

When rebasing onto 128, we broke this patch because we had to specify
an RFPTarget, and I chose RFPTarget::JSLocale.
This is the only target that can be disabled even when running RFP
instead of FFP, because it is controlled by privacy.spoof_english.

Add some notes about live-reload, discovered while investigating some
localization issues.