You don't need to configure anything specifically in RTE for this.
"When the spellcheck plugin is activated, the RTE uses dictionaries for each appropriate language. These are then selected according to the language of the website by taking either the language property of the subtree or extracting the language from the URL"
For more details, See the doc present at these locations for different versions :
6.2 : Configuring the Rich Text Editor
I was searching the spanish dictionary in the site url (https://extensions.openoffice.org). I did't find the *.zip only *.oxt file.
Can you let me know other site url where I can get the dictionary?