1. Go to Extend and install the Interface Translation module. It will prompt you to enable the Language, File, and Field modules to be installed, if they are not.
2. After the module is installed, click on Configuration. Go to the Languages page under the Regional and Language section.
3. Click on Add language in the languages overview table:

4. The Add language page provides a select list of all available languages that the interface can be translated to. Select Spanish, and then click on Add language.
5. A batch process will run, install the translation language files, and import them.
6. The INTERFACE TRANSLATION column specifies the percentage of active translatable interface strings that have a matching translation. Clicking on the link allows you to view the User interface translation form:

7. The Filter Translatable Strings form allows you to search for translated strings or untranslated strings. Select Only untranslated strings from the Search in list and click on Filter.
8. Using the textbox on the right-hand side of the screen, a custom translation can be added to Only untranslated strings. Type in a translation for the item:

9. Click on Save translations to save the modification.
10. Go to /es/node/add, and you will note that the Basic page content type description will now match your translation.