Lyx :- Package babel Error: You haven't specified a language option

I just ran into the same problem with a German document. Here is what fixed the problem without any workarounds in Ubuntu; You need to adjust this to your TeX/OS combination.

sudo apt install texlive-lang-german

Also note that the described PDF Properties workaround did not work in my case. In general, I would always first look for ways to fix the actual problem, rather than using workarounds that strip my software of its features.

Edit: On Fedora this helped: sudo dnf install texlive-babel-german

You have to install texlive-lang-all (or texlive-lang-english, texlive-lang-italian, etc.).

In the terminal:

sudo apt install texlive-lang-all

Then you have to change the preferences in LyX: Language: automatic.

It should work.