Lisbon is the name of a city, a proper noun which should be capitalized. There is no English adjective for a person or thing of or from Lisbon, but the proper adjective in Portuguese is Lisboeta.
The noun Lisbon is a proper noun, the name of a specific place.A proper noun is always capitalized.
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
Vietnamese is the proper adjective for Vietnam.
The proper adjective for Georgia is Georgian.
The proper adjective for George is Georgian.
Freudian is the proper adjective for Freud.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.