Is Spanish one of the installed languages that are send to the web server as part of the HTTP request headers?

You can check the order of the installed languages.

• Options > Content > Languages

It doesn't work, I've just choosen 'Español/España (es-es)' and now I choose 'Español (es)' but shows some websites on English on both cases.

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Edit > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Edit > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

I clean cache and cookies for the problematic pages but the problem persist. The problematic pages are ones that detects the language of the navigator, they think mozilla is on English but the configuration is on Spanish. Google chrome, opera, Mozilla for Windows and older versions of Mozilla for GNU/Linux shows these pages on Spanish.

I update to Mozilla 30 and the problem is solved. Thanks a lot.