l10n: Ensure JavaScript language packages are loaded for the user locale.

`load_script_textdomain()` had a reversed check to decide if the user locale should be loaded. This check is now replaced with `determine_local()`.

Props ocean90.

Merges [43959] into trunk.

Fixes #45465.
Built from https://develop.svn.wordpress.org/trunk@44278


git-svn-id: https://core.svn.wordpress.org/trunk@44108 1a063a9b-81f0-0310-95a4-ce76da25c4cd