I have the same problem but in German translation, it started when I updated translations for 4.3.
When re-uploading a standard 4.3 package to the site it goes back to a correct translation.
That should be from the WordPress translation itself and should already be translated if you have the Spanish installation running.
Manual re-upgrade and deleting a folder named cache seemed to do the job.
I have the same problem with a French translation. “Posted by”, “Replies”, “Older posts”, etc. are all displayed in English. Happened after updating to 3.4 and twenty eleven to 1.4.
Widgets and most other features are still in French.
All plugins and childthemes are disabled.
