string $locale )
Filters the locale ID of the WordPress installation.
The locale ID.
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return apply_filters( 'locale', $locale );
You can leverage this hook to force the language of your admin dashboard, however, you need to call it from a plugin and not from your theme, by the time WordPress gets to loading your theme it’s too late.
Example migrated from Codex:
An example of changing the locale language based on the $_GET parameter of the URL.