Add Spanish messages for id 'sp'

The id of the Spanish vocabulary is currently sp. (see spanishId.png)

Standards like ISO, GS1, and java locales use es as identifier:

• https://www.gs1.fi/en/support/synkka-support/data-pool-instructions-and-support/code-lists/language-code
• https://fr.wikipedia.org/wiki/Liste_des_codes_ISO_639-2
• http://www.oracle.com/technetwork/java/javase/javase7locales-334809.html

To keep compat, create a new messages_sp.properties file for Spanish so that translations are picked up when using this id.