The language of Orwell's Nineteen Eighty-Four is Newspeak: a thoroughly politicised and obfuscatory language designed to make coherent thought impossible by limiting acceptable word choices.

The answer depends on whether you want to help your readers or you wish to be obfuscatory or appear pompous.

In the first case, the answer is 120-gon. Everyone will know exactly what you mean.

For the second, follow the attached link.

The "proper" name is pentacontakaitetragon. Most people, including mathematicians, will use 54-gon since that effectively communicates the size of the polygon. But, if you want to appear pretentious or be obfuscatory, by all means use the Greek prefices.

The formal name is heptadecagon. Most people, including mathematicians, will use 17-gon since that effectively communicates the size of the polygon. But, if you want to appear pretentious or be obfuscatory, by all means use the Greek prefices.

The formal name is hectaicosikaihenagon. Most people, including mathematicians, will use 121-gon since that effectively communicates the size of the polygon. But, if you want to appear pretentious or be obfuscatory, by all means use the Greek prefices.