Source

File: wp-includes/l10n.php

function wp_get_installed_translations( $type ) {
	if ( 'themes' !== $type && 'plugins' !== $type && 'core' !== $type ) {
		return array();
	}

	$dir = 'core' === $type ? '' : "/$type";

	if ( ! is_dir( WP_LANG_DIR ) ) {
		return array();
	}

	if ( $dir && ! is_dir( WP_LANG_DIR . $dir ) ) {
		return array();
	}

	$files = scandir( WP_LANG_DIR . $dir );
	if ( ! $files ) {
		return array();
	}

	$language_data = array();

	foreach ( $files as $file ) {
		if ( '.' === $file[0] || is_dir( WP_LANG_DIR . "$dir/$file" ) ) {
			continue;
		}
		if ( substr( $file, -3 ) !== '.po' ) {
			continue;
		}
		if ( ! preg_match( '/(?:(.+)-)?([a-z]{2,3}(?:_[A-Z]{2})?(?:_[a-z0-9]+)?).po/', $file, $match ) ) {
			continue;
		}
		if ( ! in_array( substr( $file, 0, -3 ) . '.mo', $files, true ) ) {
			continue;
		}

		list( , $textdomain, $language ) = $match;
		if ( '' === $textdomain ) {
			$textdomain = 'default';
		}
		$language_data[ $textdomain ][ $language ] = wp_get_pomo_file_data( WP_LANG_DIR . "$dir/$file" );
	}
	return $language_data;
}