You're wrong:
* Pick any European keymap under Ubuntu, Windows, or Mac with your US keyboard
* Hold right-alt and press a key
You'll get something like ń or ą.
Bonus: Under Ubuntu, you might also have other keys remapped to let you type things like x²≠¼.
Bonus hack: Add a Greek keymap, and you can even get to things like Δx = π·y with an alt-space in between.