I'd say the proper adjective is a Japanese garden
A proper adjective is an adjective formed from a proper noun. "Arabian", "Grecian", "Japanese", "African".
Yes, if you are using it as a noun, i.e. to refer to a native of Japan, or the langauge spoken there, it is a proper noun. Otherwise it is an adjective.
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.
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.
The proper adjective for George is Georgian.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
The proper adjective for Georgia is Georgian.