Speak English or French.
Along with dozens of other countries, Canada has two official languages: English and French.
To become a citizen, you need to know just one. You don't need to be fluent, just conversational enough to make small talk, give directions, use basic grammar, and know your vocab well enough to describe yourself.
You'll send along written documents with your application, but a citizenship officer will make the final call whether your English or French is up to snuff.