Revert changes in fallback behavior

This patch set reverts the following:
* Iaaf6ccebd8c40c9602748c58c3a5c73c29e7aa4d
* Ib607a446d3499a3c042dce408db5cbaf12fa9e3d
* Ic59fd20856eb0489d70f3469a56ebce0efb3db13 (partially)

Bug 46579 comment 17 describes a desired solution. In
If88923119179924a5ec091394ccab000ade16b3e we are working on a fix, but it is
taking longer than we anticipated. There was a deployment window planned
about now, but we didn't make it. It makes sense to revert for now, and commit
a proper solution somewhere next week.

Bug: 46579
Bug: 1495

Conflicts:

	languages/Language.php
	tests/phpunit/includes/cache/MessageCacheTest.php

Change-Id: Iac7ac4357dd80e8cdb238a6a207393f0712b3fe5