He asked how King Achashverosh's guards Bigtan and Teresh were able to plot, and thought they'd be able to carry out, the murder of the king.
Well, my son explained that Bigtan and Teresh were not from Shushan. They were from a far away country, and spoke a little known language, Tarsi. They could plot to their heart's content, because no one would understand. Or so they thought.
Mordechai HaYehudi, Esther's uncle, was one of the exiled Jews from Eretz Yisrael. While he still lived in Israel, Mordechai was a member of the Sanhedrin.
Since judges of the Sanhedrin were not allowed to accept the testimony of witnesses through an interpreter, all the members of the Sanhedrin had to know the 70 languages of the world.
Because of that, even thought Bigtan and Teresh spoke in a very rare language, Mordechai was able to understand them and interpret their develish intentions. B"H.