The people that have always lived in an area, regardless of the languages, cultural changes or changes in religion remain the native people. Just because native americans may have become Roman Catholics, may speak Spanish and may have adopted much of the Hispanic (Spanish) culture, does not somehow change their status as the native people of America.
By definition, Arab Muslims have NOT "always lived there".
By definition, Spanish cultures are NOT the indigenous cultures of the Americas.
By definition, I (Canadian of Irish/Scots/German descent) am from an immigrating (invading) culture and am not First Nations. Nor will my descendants EVER be.
By definition invading, colonizing cultures are not First Nations cultures.
The PURPOSE of identifying and protecting First Nations cultures is to preserve the cultures of those who were invaded and conquered.
Can a First Nations person learn to speak a foreign language and still be First Nations? Duh. Of course, they can. Can a First Nations culture entirely adopt an invading culture so that there is no trace of their First Nations culture and still be First Nations. Uh. No.
We invaded a really, really long time ago does not confer First Nations status on people. Though it might give them claims to sovereignty and self-determination.