Backup intl.locale.requested in 11.5.x

With multi-lingual builds we are going to make intl.locale.requested empty, to match OS locale.

However, we could use a 11.5.7 (localized) release to make it a user pref (in prefs.js).

In this way, when user will switch to the multi-lingual 12.0, they will have their preferred language saved as a user pref, and will override our new value of intl.locale.requested.

If this works as expected, users updating from 11.5.7 to 12.0 will not lose their language preference.