"Marcus is American."
Translation:Marcus est Americanus.
I don't know if the sentence or the discussion has been reworked or deleted, because the link doesn't work for me.
So, I don't know what has been said, but capitalization of nationalities is not one of the Latin rules, it seems anglicized Latin: adding rules specific to the English language in Latin.
I think they could include the adjective «septentrionalis» before americanus to denote someone from North America. americanus in this context could be South American or Central American.
I think this scheme below is a more culturally neutral one, and also more respectul to Latin American duolinguists:
North American: americanus septentrionalis
Central American: americanus centralis
South American: americanus australis aut americanus meridianus, nec confundātur cum vocabulo «australis», quod significare potest australianus, persona ex natione sita ad austrum. Praefero tamen primum vocabulum.
I'm sure this has been mentioned elsewhere but I'll say it anyway. The use of America (and associated names) is ridiculous given that the word wasn't coined until a millennium after the end of the Roman empire. What about using just Eboracum for York. Also Londinium - London, Britannia - Britain, Belgica - Belgium, Carthago - Carthage, Alexandria, Aegyptus - Egypt and so on? Learning Latin is not just about learning a language. Its much more than that; about the history and geography of an ancient empire.