You are very right. After learning the third, you start to feel a glimpse of some more universal structure in your brain, that pinpoints meaning to expression in languages you know. So you can tell "this cant be said in English, it is too Spanish", or something like that. And it is very useful to learn at least one Eastern Asian language with non-latin script, to get full experience of this kind. Your own depths are unfathomable, and every language pulls the cover, unraveling the abyss of your own mind.