Kansainvälinen appears in our example as kansainväliset, its plural form. It’s in the plural because it modifies suurmarkkinat which is a plural noun.
It is a calque (word for word translation) of the word international.
Let’s translate the other parts of the poster.
Ruokaherkkuja ja erikoistuotteita ympäri maailma means ”Food delicacies and specialty products from around the world.”
Narinkkatori is the location. The International Market the poster talks about will be held in ”Narinkka Square” in Helsinki. It takes place August 8th through 11th.