It doesn’t matter where George Romney was born. Was he an American citizen when Mitt was born?
If the argument that only a "natural born citizen" (born within the US territory, and born from 2 citizen parents) can be President, it will matter.
This was an issue in 1967-1968, when the self-described "brainwashed" George Romney ran for President.
The 14th Amendment extends citizenship to persons born on American-held soil, but George Romney was born in 1907 in Mexico.