I've noticed something interesting in regards to speaking in Japanese. Even though I generally speak to everybody with the same vocabulary (the very little I know), only some Japanese can completely understand what I'm saying. Most people I've met can understand what I'm saying nearly all the time. Once in a while, however, I meet someone who I have lots more difficulty communicating with.
When meeting new Japanese folks, I always make it a point to teach them the most important words in English: that is, all the curse words.