You should at all costs avoid making any changes to Drupal core.

• Any changes you make to Drupal core will also need to be added again when you ugrade your project with a security release
• or update your project to the next version of Drupal
• Your system may become unstable as you are changing the framework within which modules operate
• You may create security vulnerabilities

• Using the Drupal hook system it's possible to override most of Drupal's processing
• The theme system gives you the possibility of overriding all content being output
• Locale can be used to change the wording of Drupal's interface