Takahiro,
"
Canada is an English-speaking country" is correct. The word
speaking is acting as an
adjective describing the word "country". We use the present participle form (-ing form) when the word has an
active meaning.
Oh, I had to check my English Usage book, so thanks for teaching me 
To use "spoken", you could make a passive sentence:
"English is
spoken in Canada."
It's passive because we don't tell who the actors are, or
who exactly speaks English in Canada. In the case we know it is people and not, um, dogs or polar bears
