"My American friend is buying a bicycle."
Translation:Mój amerykański przyjaciel kupuje rower.
The order of noun vs. adjective seems to be flexible in general but Mój przyjaciel amerikański kupuje rower was not accepted - after the possessive pronoun the adjective has to precede the noun?
Interesting; thanks for the link. According to this, being American should be a temporary or incidental property of the friend if the adjective precedes the noun.
I would say that "American" is not a property that defines the friend. His nationality does not automatically make him look and/or behave differently from others. It's more of a background info.