This is the 6634th most frequent Polish word.
dominikana
Dominikana: Dominican Republic.
Here, 'Dominikana' is used as the proper noun referring to the country, the Dominican Republic.
Dominikana to piękny kraj na Karaibach.
The Dominican Republic is a beautiful country in the Caribbean.
'Dominikanie' is the locative case of 'Dominikana', used here to express location or place.
Spędziłem wakacje na Dominikanie.
I spent my vacation in the Dominican Republic.
Again, 'Dominikana' functions as a proper noun, specifically in the nominative case as the subject of the sentence.