Avoid PHP notices when trying to update translations which are already up to date

[ocean90]

This is bit of a race condition:

• Make sure that you have outdated translations and auto-updates for translations are working.
• Go to wp-admin/update-core.php.
• Now do something else.
• In the meantime WP has updated the translations in the background.
• Click the Update Translations button.

Expected: No PHP notices
Actual: PHP notices:

NOTICE: wp-admin/includes/class-wp-upgrader.php:2204 - Trying to get property of non-object
Language_Pack_Upgrader->bulk_upgrade, Language_Pack_Upgrader_Skin->before, Language_Pack_Upgrader->get_name_for_update
NOTICE: wp-admin/includes/class-wp-upgrader-skins.php:811 - Trying to get property of non-object
Language_Pack_Upgrader->bulk_upgrade, Language_Pack_Upgrader_Skin->before

[Presskopp]

Possible coincidence!?

wp-admin/includes/class-wp-upgrader.php
Line 1980:

$this->strings['up_to_date'] = __( 'The translation is up to date.' ); // We need to silently skip this case

[ocean90]

In 37635:

Upgrader: Don't use Language_Pack_Upgrader_Skin::before() and Language_Pack_Upgrader_Skin::after() when translations are up to date.

Avoids a PHP notice since Language_Pack_Upgrader_Skin::before() tries to access a non-existent update.
Also, change the up_to_date string to use the plural form because translations are only updated in bulk mode.

Fixes #36615.