I18N: Fix passing singular/plural strings to JS.

This is a temporary solution while waiting for full I18N support in JS.

Props ocean90, dd32, westonruter, Mirucon for testing.
Amends [41376], [41721], [41389],
See #20491.
Fixes #42510 for trunk.

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git-svn-id: https://core.svn.wordpress.org/trunk@41994 1a063a9b-81f0-0310-95a4-ce76da25c4cd