[wp-trac] [WordPress Trac] #61206: Allow to filter the returned value of `WP_Textdomain_Registry::get()`

[WordPress Trac]

#61206: Allow to filter the returned value of `WP_Textdomain_Registry::get()`
-------------------------+-----------------------------
 Reporter:  Chouby       |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  I18N         |    Version:  6.1
 Severity:  normal       |   Keywords:
  Focuses:               |
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 The context is locale fallback as implemented by @swissspidy in the
 [Preferred languages https://github.com/swissspidy/preferred-languages/]
 plugin or a similar feature in Polylang Pro.

 When a translation file in the current locale is not found for a plugin or
 theme, the Preferred languages plugin allows to fallback to a different
 locale.
 This is especially useful when a language variant has less translations
 available than another. For example fr_BE has less language packs
 available than fr_FR.

 The feature is easy to implement with existing WP filters for plugins or
 themes loading explicitely their translation using
 `load_plugin_texdomain()` or `load_theme_texdomain()`.

 This is trickier when the translations are loaded by
 `_load_textdomain_just_in_time()`. The reason can be found in the
 implementation of the function. If no translation file is found by
 `WP_Textdomain_Registry`, then [the function simply bails early
 https://github.com/WordPress/wordpress-develop/blob/6.4.3/src/wp-
 includes/l10n.php#L1301-L1304].
 There is currently no filter in `WP_Textdomain_Registry` or
 `_load_textdomain_just_in_time()` which could allow to provide an
 alternative translation file.

 The Preferred languages plugin works around the limitation by using the
 `gettext` filter family. This means that the code to implement the locale
 fallback is typically executed thousands of times (once per call to the
 `__()` function family) when we would ideally want to execute it only once
 per text domain.

 Adding a filter to modify the returned value of
 `WP_Textdomain_Registry::get()` would allow to implement locale fallbacks
 more efficiently.

 Related: #28197

 For references:
 https://make.wordpress.org/core/2024/05/07/merge-proposal-preferred-
 languages/
 https://github.com/swissspidy/preferred-languages/issues/1049

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Ticket URL: <https://core.trac.wordpress.org/ticket/61206>
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