Filter language labels, not the configured locale list

There is no general public filter that replaces pom_translate_get_available_languages(). Enabled locales come from site settings.

pom_translate_language_custom_labels filters the locale-to-label map used by selectors:

add_filter( 'pom_translate_language_custom_labels', static function ( array $labels ): array {
    $labels['en_GB'] = 'International';
    return $labels;
} );

Use settings to add or remove locales. Do not inject a label for an unconfigured locale and expect routing, translation storage, or hreflang to support it.