Hello,
I found a solution for this problem. In my opinion, there is an error with the system you use to translate the theme to different languages. By deafult, if you want wordpress and the theme to be in other language (just one), it is not needed any plugin (WPML or similar). You just have to define the language in wp-config.php and have the right po files for the theme.
With the structure of your theme, wordpress does not know which file is the right one and cannot show the theme in the right language. There are several directories with language files and I suppose that you should tell wordpress where the right one is. I found this page that tells you must define the language directory in functions.php.
MY SOLUITON
I installed this plugin CodeStyling Localization Preserver that forces wordpress to read the language file from /wp-content/languages/themes . So in this way, when I save the Spanish .po file in Codestyle Localization plugin, it copies the file to that directory.
This is “a plugin of a plugin” and Codestyle Localization plugin is needed.
I would say that I have translated at least 10 themes before and I have never had this problem before. So It is strange that no other have had this problem before.