It seems there are some configuration problems (webserver or PHP) regarding the language packs, that even affect older versions of CeresCP (like the one you are now using), which causes text that contains special characters (like è, é, ñ, ä) to not display anymore.
If you have the spanish language pack active and receive an empty message box when trying to login, it seems to be mostly the "Incorrect Code." message that's not displayed (tested on latest version).
I'll take a look at the general webserver/PHP configuration and the code and see if I can find a solution for this, but for now the only workaround is to just use the english translation.
Update:
Okay, I just figured out the problem. Since PHP 5.4 htmlentities, which is used for encoding the language pack texts, uses UTF-8 as default encoding, but we're having an ISO-8859-1 string, so it just returned an empty string.
This has now been fixed in the newest version of CeresCP. Make sure you use the official SVN repository (or my GIT repo) to download from, instead of downloading from the download section, containing outdated pre-packed files.