`wp_version_check()`, `wp_update_plugins()` and `wp_update_themes()` are all originally hooked to the `upgrader_process_complete` action with zero arguments passed to them. Zero arguments should be passed when re-adding them after translation updates, otherwise the sky will fall.
Merge of [38415] to the 4.6 branch.
Props ionutst, gitlost, swissspidy.
Fixes#37731.
Built from https://develop.svn.wordpress.org/branches/4.6@38475
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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.
Built from https://develop.svn.wordpress.org/trunk@37635
git-svn-id: http://core.svn.wordpress.org/trunk@37603 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Jeremy Felt
Dominik Schilling
Drew Jaynes