This is the 4729th most frequent Dutch word.
eupen
"Eupen" is not a Dutch word; it is the name of a town in Belgium.
The word 'Eupen' here represents the name of a city in Belgium, thus used as a proper noun.
Ik heb een boek gelezen over de stad Eupen.
I read a book about the city Eupen.
Same as before, 'Eupen' refers to the city and provides context to the cultural aspect of language there.
In Eupen spreekt men een variant van het Duits.
In Eupen, one speaks a variant of German.
'Eupen' remains the name of the city, describing a characteristic of its inhabitants.