I’m loving this German/Dutch exchange!
Nothing really, unless you count pig latin.
I’ve got about a kindergartener’s vocabulary in American Sign Language. I took three years of high school Spanish which allows me to read basic Spanish, but understanding the spoken language is beyond my ability. It’s just about enough to get by comfortably as a tourist.
I know some Elvish, too – Quenya and Sindarin – but they aren’t anything close to complete languages as far as vocabulary, and the grammar isn’t particularly standardised.