This is true but no one wants to accept the consequences of this nihilistic view of things, postmodernism is dead and it killed itself.
I wouldn't say that this is nihilistic, merely despairing (in a Schaefferian sense).
As for the language-game, you can, in fact, judge others on the basis of a language-game. What you cannot do is judge another language-game. Thus, the liberal language-game could denounce Hitler, but it could not judge Nazism. It's an odd position, but not technically inconsistent.
Also I wouldn't be so cold hearted with a lay person on having a theory of ethics but I would ask for reasons why they held such beleifs.
But what you are finding there is the synthetic thread that they think binds the particulars together. The particulars are more basic than the theory--the purpose of the theory is to explain the particulars. If the person happens not to need a theory, there is nothing that compels him or her to do so.
This sounds a little empiricist in regards to beleif formation to me but I don't at all that you are not an empiricist.
Is it so wrong to say that certain basic beliefs are empirical? I also count as basic my beliefs that 2+2=4, or that God is there.
I would just ask them why?
Because they don't share your assumptions. You have to admit the premises, or the conclusion is uncompelling.