Ensures that string lookup in MO files only uses the singular string.
This matches expected behavior with gettext files and 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. Maintains backward compatibility and feature parity with the pomo library and the PHP translation file format.
Replaces [57386], which was reverted in [57505], with a more accurate and performant solution.
See #59656.
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This introduces a more lightweight library for loading `.mo` translation files which offers increased speed and lower memory usage.
It also supports loading multiple locales at the same time, which makes locale switching faster too.
For plugins interacting with the `$l10n` global variable in core, a shim is added to retain backward compatibility with the existing `pomo` library.
In addition to that, this library supports translations contained in PHP files, avoiding a binary file format and leveraging OPCache if available.
If an `.mo` translation file has a corresponding `.l10n.php` file, the latter will be loaded instead.
This behavior can be adjusted using the new `translation_file_format` and `load_translation_file` filters.
PHP translation files will be typically created by downloading language packs, but can also be generated by plugins.
See https://make.wordpress.org/core/2023/11/08/merging-performant-translations-into-core/ for more context.
Props dd32, swissspidy, flixos90, joemcgill, westonruter, akirk, SergeyBiryukov.
Fixes#59656.
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