Add support for using only PHP translation files

[swissspidy]

In WordPress 6.5 we're adding support for PHP translation files. They still require accompanying .mo and .po files. However, it doesn't have to be this way!

It should be possible for projects (plugins, themes, and core) to only use .l10n.php files.

A few areas that would need changing:

wp_get_installed_translations()

wp_get_installed_translations() is used for getting the list of installed translations for plugins/themes/core when updating language packs and also when uninstalling plugins/themes (to remove the translations again).

Language_Pack_Upgrader

\Language_Pack_Upgrader::check_package() checks whether there are .mo and .po in the language pack. It should also support language packs with only a .l10n.php file.

load_textdomain() / load_default_textdomain()

Not sure about this one.

Right now you can pass an .mo file path, even though the file doesn't exist, and WordPress will load the .l10n.php counterpart. So technically no change is required. However, what if you want to directly pass an .l10n.php file path instead?

1. Scan for PHP files in wp_get_installed_translations(). So if only a PHP translation file exists, updates still work as expected. Also has the benefit of using $wp_textdomain_registry->get_language_files_from_path() instead of scandir, so the initial file list is cached.
2. Support language packs that contain only PHP translation files but no mo/po.

Trac ticket: https://core.trac.wordpress.org/ticket/60554

[swissspidy]

In 58061:

I18N: Improve support for using only PHP translation files.

This builds on top of the PHP translation file support added in WordPress 6.5, improving the behavior for projects using solely .l10n.php translation files and no .mo. and .po files.

Updates wp_get_installed_translations(), which is used when updating language packs and when uninstalling plugins/themes (to remove the translations again), to look for PHP translation files and read metadata from them. Additionally, the file lookup is now cached thanks to using WP_Textdomain_Registry.

Updates Language_Pack_Upgrader::check_package() to allow language packs that only contain PHP translation files. While WordPress.org continues to serve .mo and .po files, third-party services might want to only use the PHP file format.

Props swissspidy.
Fixes #60554.

[swissspidy]

This can be added to the general i18n dev-note for 6.6.

[swissspidy]

In 58062:

I18N: Actually add all the files for [58061], not just the test fixtures.

Improve support for using only PHP translation files.

This builds on top of the PHP translation file support added in WordPress 6.5, improving the behavior for projects using solely .l10n.php translation files and no .mo. and .po files.

Updates wp_get_installed_translations(), which is used when updating language packs and when uninstalling plugins/themes (to remove the translations again), to look for PHP translation files and read metadata from them. Additionally, the file lookup is now cached thanks to using WP_Textdomain_Registry.

Updates Language_Pack_Upgrader::check_package() to allow language packs that only contain PHP translation files. While WordPress.org continues to serve .mo and .po files, third-party services might want to only use the PHP file format.

See #60554.

https://core.trac.wordpress.org/changeset/58061
https://core.trac.wordpress.org/changeset/58062