The adjective form of Buddhism is Buddhist, using the -ist ending. As an example, "Buddhist teachings began in ancient India and quickly spread across its Asian neighbors." Teachings are the noun, and Buddhist is the adjective in that sentence. More rarely, "Buddhistic" is used.
The proper adjective for the noun Buddhism is Buddhist (for example, Buddhistphilosophy).
The word Buddhist is also a proper noun, a word for a follower of Buddhism.
Vietnamese is the proper adjective for Vietnam.
The proper adjective for the proper noun 'Celt' is Celtic.Please note that a proper noun and a proper adjective is always capitalized.
Ghanian is the proper adjective for Ghana.
Since spice isn't a proper noun, it can't have a proper adjective. The adjective of spice is spicy.
Since writer is not a proper noun, it doesn't have a proper adjective.
The proper noun for Tibetan is Tibet.
Tibetan
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.