Lyx: (package babel error) you don't have specified a language.

I had this issue last night after to upgrade my Ubuntu 13.04 to 13.10. This is was really annoying because I did not modify anything on Lyx so I had to take some time to what it happened. All you have to do is to install the language package from Synaptic Manager on your Ubuntu. In my case was spanish language package but you can download the entire language package (400 MB more less).