Different countries and federations have different rules as far as foreign nationals playing on National teams. From what I have gleaned, you generally need to prove to be second or third generation.
In my daughter's case:
My Mother was born in England- therefore my DD would be ok in the UK
On my Father's side- my Great, Great Grandparents came over from Germany-no good(nein, ist verboten) Great Grandparents on the Czech side(don't think they have a team)
Wife's side- Great Grandmother was born there, plus well over 100 relatives to attest for her(that actually can help- so Bravo Italia
There is a caveat- if someone does declare dual citizenship- it can actually make it difficult to get any government clearances(secret, top secret etc...) Not a problem for most people, but in many lines of work it can pose a problem
One exception to that is Puerto Rico- since people from Puerto Rico(or any US Territory) are US citizens, playing for those "National" teams pose no such issues



























