"The girl is American."
Translation:La niña es americana.
The reason is that señorita is used to address older people most of the time, where niña defines itself as "girl", where a girl is usually interpreted as young.
Now, the common argument would be that señorita defines to "miss", which could also relate to a young female. However, there are 2 things wrong there. First of all, "miss" is most commonly used to address ages such as teenagers/young adults, while "girl" is most typically used to define ages such as 1-10. Second, as this is a regional definition, it is usually not accepted as correct.
I hope this helps!