Respect users locale

[carstingaxion]

Description of the Change

Closes #746

How to test the Change

BEFORE: Steps to reproduce the problem

1. Have English selected in Settings > General as Site language.
2. Edit your user profile, choose a different language for your user and save!
3. See the main admin menu changed into your users language.
4. Go to Plugins and toggle the activation of the plugin on and off.
   A. Plugin not activated: Admin Bar is translated into your users language
   B. Plugin activated: Admin Bar is NOT translated, shows in English

AFTER (applying this PR): Steps to see working solution

1. Have English selected in Settings > General as Site language.
2. Edit your user profile, choose a different language for your user and save!
3. See the main admin menu changed into your users language.
4. Go to Plugins and toggle the activation of the plugin on and off.
   A. Plugin not activated: Admin Bar is translated into your users language
   B. Plugin activated: Admin Bar is (now also) translated into your users language

Changelog Entry

Fixed - Do respect the logged-in users locale within the Admin UI.

Credits

Props @carstingaxion

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[what-the-diff]

PR Summary

• Localization Enhancement in User Time Zone Selection
  This PR introduces an upgrade in the timezone selection functionality. Previously, the system presumed a standard timezone for all users. With the changes made, it now accommodates the user's locale (which is their geographical location) to provide more accurate timezone options. This can lead to an improved user experience since the application's time settings will align better with the user's local time.

[carstingaxion]

WordPress core (wp-admin/options-general.php L345-L371) is even more accurate and takes timezone offset into account too:

$current_offset = get_option( 'gmt_offset' );
$tzstring       = get_option( 'timezone_string' );

$check_zone_info = true;

// Remove old Etc mappings. Fallback to gmt_offset.
if ( str_contains( $tzstring, 'Etc/GMT' ) ) {
	$tzstring = '';
}

if ( empty( $tzstring ) ) { // Create a UTC+- zone if no timezone string exists.
	$check_zone_info = false;
	if ( 0 === (int) $current_offset ) {
		$tzstring = 'UTC+0';
	} elseif ( $current_offset < 0 ) {
		$tzstring = 'UTC' . $current_offset;
	} else {
		$tzstring = 'UTC+' . $current_offset;
	}
}?>
<th scope="row"><label for="timezone_string"><?php _e( 'Timezone' ); ?></label></th>
<td>

<select id="timezone_string" name="timezone_string" aria-describedby="timezone-description">
	<?php echo wp_timezone_choice( $tzstring, get_user_locale() ); ?>

where GatherPress just states:

$timezones_raw = explode( PHP_EOL, wp_timezone_choice( 'UTC', get_user_locale() ) );

Maybe a chance for later improvement, maybe essentially needed; I don't know.

[mauteri]

@carstingaxion tested this and works as you described. merging! nice job on this!