You just have to enable the following setting and pages that are not translated will default back to your own "default".
Check the Multilingual section on this info page: https://docs.kentico.com/k10/configuring-kentico/managing-sites/configuring-settings-for-sites/settings-content
Combine with default culture - Indicates if the default culture version of pages should be offered for pages that are not available in the currently selected language.