1. The Ossn 4.6 you are running comes with Spanish already included in the core package - hence there's no need to install an extra language pack.
2. You have to select the language of your choice from your Ossn Admin panel by choosing Site Settings -> Basic -> Default language. Maybe you have to disable the cache before doing so and re-enable it afterwards.
3. In opposite to other languages on your system I'm getting an error 500 when trying to run your currently installed Spanish language file manually - it seems to be corrupted!

So please overwrite all ossn.es.php files that came included with the package you mistakenly installed with the original files included in the Ossn 4.6 archive and switch to "Spanish" afterwards as said under 2.