How to create a Spanish version of node which is in English and that Spanish version of node should contain all fields from English content except one field. That one field should translate to Spanish Lang
Besides adding Spanish language in your site Regional settings (with Locale module from Drupal core) you need to install the Internationalization module.
Internationalization module comes with a lot of submodules. Read about them in the documentation: Submodules and see if some of them are useful to your multilingual setup.
The Synchronize translations submodule is the one you want where you can set that some fields are the same for all languages.