is_user_logged_in() is a pluggable function, and loaded after plugins are loaded. If a plugin calls __() too early, is_user_logged_in() is missing and WordPress will fatal. get_user_locale() already handles this scenario for us, so it's safe to rely on exclusively.
Props danielbachhuber.
Merges [43846] to trunk.
See #44758.
Fixes#45235.
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git-svn-id: http://core.svn.wordpress.org/trunk@44011 1a063a9b-81f0-0310-95a4-ce76da25c4cd