"The girl is American."
Translation:La niña es americana.
Capitalization is used in Spanish, but only at the beginning of sentences and for Proper Nouns (the specific name for a particular person, place, or thing)
As a result, nationalities are not capitalized in Spanish, but countries are.
- The girl is from the United States of America. She is American
- La niña es de los Estados Unidos de América. Ella es americana
Duolingo doesn't check capitalization.
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!