Remove `load_theme_textdomain()` calls from default themes

[swissspidy]

Since WordPress 4.6 introduced just-in-time translation loading, themes and plugins no longer need to manually call load_theme_textdomain/load_plugin_textdomain, unless they are on a version prior to 4.6.

Still, for some reason the default themes before Twenty Twenty-Two still use load_theme_textdomain, sometimes even pointing to a /languages folder that doesn't actually exist.

Related: #34884

Suggestion:

1. Remove load_theme_textdomain() call from Twenty Seventeen and up, since these themes all require at least WordPress 4.7
2. On older default themes, only call load_theme_textdomain() conditionally depending on the WordPress version.

[spacedmonkey]

If we make this change, I would like to see a dev note, so it means that others do not make the same mistake.

[swissspidy]

FWIW, we already had a dev note for this 7 years ago: ​https://make.wordpress.org/core/2016/07/06/i18n-improvements-in-4-6/

[audrasjb]

Nice catch, let's move this to milestone 6.3.

[spacedmonkey]

I think it is worth writing a blog post about this. I had no idea about this change. I think flagging and the performance impact it might have would go a long way to get themes to stop using these functions.

Looking at the theme directory, seems to be lots of plugins there that use it. See ​https://www.wpdirectory.net/search/01H0Q50AEDHDWKN0TFDZ3BJZ5M

Including storefront, Astra and OceanWP, three of the largest themes in the repo.

[swissspidy]

Sure we can mention this somewhere as a reminder. Not really a performance impact though

Remove explicit call to load_theme_textdomain() after twentyseventeen theme since they all require WordPress version 4.6 (just in time translation loading introduced) and above. Add a wp_version conditional check for themes before twentyseventeen for a call to load_theme_textdomain() to be included.

Trac ticket: https://core.trac.wordpress.org/ticket/58318

[spacedmonkey]

@swissspidy Do you plan to own / commit this ticket?

[swissspidy]

In 55929:

Bundled Themes: Remove load_theme_textdomain() calls from default themes.

Since WordPress 4.6 introduced just-in-time translation loading, themes and plugins no longer need to manually call load_theme_textdomain/load_plugin_textdomain, unless they are on a version prior to 4.6.

This change removes the load_theme_textdomain() call from Twenty Seventeen and up, since these themes all require at least WordPress 4.7.
On older default themes, load_theme_textdomain() is called conditionally depending on the WordPress version.

Props piyushtekwani.
Fixes #58318.

Committed in https://core.trac.wordpress.org/changeset/55929