Fx60: Fix default locale prefs

An empty string for `intl.locale.requested` means to use the OS locale,
whereas if it's absent the default locale (English) is used.

`intl.regional_prefs.use_os_locales` isn't actually a replacement for
`intl.locale.matchOS` and should be false by default. See [1] for a
description.

[1] https://firefox-source-docs.mozilla.org/intl/locale.html

assets/prefs.js

@@ -13,8 +13,8 @@ pref("network.prefetch-next", false);
 // Let operations run as long as necessary
 pref("dom.max_chrome_script_run_time", 0);
 
-// Use OS locale
-pref("intl.regional_prefs.use_os_locales", true);
+pref("intl.locale.requested", '');
+pref("intl.regional_prefs.use_os_locales", false);
 
 // Fix error initializing login manager after this was changed in Firefox 57
 // Could also disable this with MOZ_LOADER_SHARE_GLOBAL, supposedly