Nine cities, each city differs from every other city on the list. Find each cities defining feature and that will give you another city. What is that city?
AMSTERDAM ..... Congratulations 'PotatoBoy'....... another CHALLENGE solve for you (I think 7 now): I would like to know how you first realised the challenge had to do with language?....... anyway, your 'Honorary Gold Medal' is on its way..... congrats. again: PS did the Cat's 'Tongue' give you a clue?
It said to determine every city's defining feature, so I started with Panaji, the odd one in the group because I had never heard of it. When I saw it is part of Goa, I immediatly thought it was probably one of the very few cities in India where most of the population speaks Portuguese. And then I realised each city on your list spoke a different language. So the language initials gave me AMSTERDAP. A quick google search showed me Marathi is also a major language in Panaji and then it all fell in place.