sion_from_header( $this->headers['plural-forms'] );
			$this->plural_forms = $this->make_plural_form_function( $expression );
		}

		if ( is_callable( $this->plural_forms ) ) {
			/**
			 * Plural form.
			 *
			 * @var int $result Plural form.
			 */
			$result = call_user_func( $this->plural_forms, $number );

			return $result;
		}

		// Default plural form matches English, only "One" is considered singular.
		return ( 1 === $number ? 0 : 1 );
	}

	/**
	 * Returns the plural forms expression as a tuple.
	 *
	 * @since 6.5.0
	 *
	 * @param string $header Plural-Forms header string.
	 * @return string Plural forms expression.
	 */
	protected function get_plural_expression_from_header( string $header ): string {
		if ( preg_match( '/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches ) ) {
			return trim( $matches[2] );
		}

		return 'n != 1';
	}

	/**
	 * Makes a function, which will return the right translation index, according to the
	 * plural forms header.
	 *
	 * @since 6.5.0
	 *
	 * @param string $expression Plural form expression.
	 * @return callable(int $num): int Plural forms function.
	 */
	protected function make_plural_form_function( string $expression ): callable {
		try {
			$handler = new Plural_Forms( rtrim( $expression, ';' ) );
			return array( $handler, 'get' );
		} catch ( Exception $e ) {
			// Fall back to default plural-form function.
			return $this->make_plural_form_function( 'n != 1' );
		}
	}

	/**
	 * Imports translations from another file.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Translation_File $source Source file.
	 * @return bool True on success, false otherwise.
	 */
	protected function import( WP_Translation_File $source ): bool {
		if ( null !== $source->error() ) {
			return false;
		}

		$this->headers = $source->headers();
		$this->entries = $source->entries();
		$this->error   = $source->error();

		return null === $this->error;
	}

	/**
	 * Parses the file.
	 *
	 * @since 6.5.0
	 */
	abstract protected function parse_file();

	/**
	 * Exports translation contents as a string.
	 *
	 * @since 6.5.0
	 *
	 * @return string Translation file contents.
	 */
	abstract public function export();
}
<br />
<b>Fatal error</b>:  Uncaught Error: Class 'WP_Translation_File' not found in /home/yurtglob/public_html/wp-includes/l10n/class-wp-translation-file-mo.php:15
Stack trace:
#0 /home/yurtglob/public_html/wp-settings.php(123): require()
#1 /home/yurtglob/public_html/wp-config.php(97): require_once('/home/yurtglob/...')
#2 /home/yurtglob/public_html/wp-load.php(50): require_once('/home/yurtglob/...')
#3 /home/yurtglob/public_html/wp-blog-header.php(13): require_once('/home/yurtglob/...')
#4 /home/yurtglob/public_html/index.php(17): require('/home/yurtglob/...')
#5 {main}
  thrown in <b>/home/yurtglob/public_html/wp-includes/l10n/class-wp-translation-file-mo.php</b> on line <b>15</b><br />
<br />
<b>Fatal error</b>:  Uncaught Error: Class 'WP_Translation_File' not found in /home/yurtglob/public_html/wp-includes/l10n/class-wp-translation-file-php.php:15
Stack trace:
#0 /home/yurtglob/public_html/wp-includes/load.php(1518): require_once()
#1 /home/yurtglob/public_html/wp-includes/class-wp-fatal-error-handler.php(176): wp_load_translations_early()
#2 /home/yurtglob/public_html/wp-includes/class-wp-fatal-error-handler.php(156): WP_Fatal_Error_Handler-&gt;display_default_error_template(Array, false)
#3 /home/yurtglob/public_html/wp-includes/class-wp-fatal-error-handler.php(60): WP_Fatal_Error_Handler-&gt;display_error_template(Array, false)
#4 [internal function]: WP_Fatal_Error_Handler-&gt;handle()
#5 {main}
  thrown in <b>/home/yurtglob/public_html/wp-includes/l10n/class-wp-translation-file-php.php</b> on line <b>15</b><br />