Ensures that looking up a singular that is also used as a pluralized string works as expected.
This improves compatibility for cases where for example both `__( 'Product' )` and `_n( 'Product', 'Products’, num )` are used in a project, where both will use the same translation for the singular version.
Although such usage is not really recommended nor documented, it must continue to work in the new i18n library in order to maintain backward compatibility and maintain expected behavior.
See #59656.
git-svn-id: https://develop.svn.wordpress.org/trunk@57386 602fd350-edb4-49c9-b593-d223f7449a82
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