Add warnings if loading translations too early

[swissspidy]

In my quick local testing I already identified a few plugins doing load_plugin_textdomain() wrong:

• RTL Tester
  • Not really maintained anymore anyway
• Two Factor
  • Rely on just-in-time translation loading two-factor#608
• Elementor
  • Tweak: Rely on just-in-time translation loading [ED-14578] elementor/elementor#27200
• Litespeed Cache
  • Rely on just-in-time translation loading litespeedtech/lscache_wp#738

Plugins doing just-in-time loading wrong:

• WooCommerce
  • Prevent translations from being loaded too early woocommerce/woocommerce#47113
• BuddyPress
  • Their theme integration also causes Twenty Twenty-Four translations to be loaded too early

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To-do:

• Improve wording for the _load_textdomain_just_in_time case, mentioning that it's typically caused by unintended side effects in a plugin or theme.
• What about themes? They usually do stuff in after_setup_theme, which runs right before init and current user setup. Probably all classic themes will be affected by this.

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Strangely, when one of these plugins and Query Monitor is active, there is some sort of infinite loop happening and the site crashes with a 500. @johnbillion Any thoughts why QM might run in circles here?

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Trac ticket: https://core.trac.wordpress.org/ticket/44937

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[swissspidy]

Looking into warnings emitted by themes, such as twentytwentyone.

Some observations:

• Themes usually trigger just-in-time loading on after_setup_theme
• In the admin, the current user is set up right after setup_theme because of the load_default_textdomain() call in wp-settings.php
• Checking for after_setup_theme to emit a warning might be a better solution

I am concerned about calling __() from within the load textdomain functions though.

At this point determine_locale() will already have been called, and the current user thus setup. So I don't see any other side effect happening because of this.

[swissspidy]

@ocean90 @SergeyBiryukov @johnbillion Is this something one of you could provide feedback on? I think it's a nice safeguard that could still land before the beta, but it also might be a bit noisy.

[johnbillion]

This could be noisy but it might be interesting to see if any feedback about that is raised during beta.

[swissspidy]

Committed in https://core.trac.wordpress.org/changeset/59127