Check the 'Usage' instructions on the website ... where it says ...
NOTE:
By default, only the image player is loaded. If you need to play other types of content, use the players option to specify which players you'd like to load.
You haven't, so only the default 'img' player is loaded and therefore your images will display, but your 'inline' content won't. Admittedly, the 3.0b website doesn't mention 'inline' explicitly as a player option, but a quick browse of the forum would turn up several discussions that deal with it. And, as I mentioned, in the code itself it actually seems to treat 'inline' and 'html' the same way. I've tried this on my own site, and 'html' works fine for (my) 'inline' content.
I've raised on the forum what I think is a bug in the use of 'inline'; it didn't work unless I also specified the 'html' player, leading me to suspect there's a bug in the 3.0b code somewhere. So, if 'inline' still doesn't work, try 'html' instead (or as well).
<script type="text/javascript">
Shadowbox.init({
language: "es",
players: ["html"]
});
</script>