Introduced in [29630], this admin notice informs administrators (or network administrators on multisite installs) that the `WPLANG` constant is no longer needed when the locale returned by `get_locale()` does not match. After 5 years, the notice is being removed to avoid causing confusion for non-technical users.
The `_deprecated_argument()` call will persist to inform developers of the issue within log files.
Props flixos90, iworks, ocean90.
Fixes#39675.
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