"A German visits an American in the United States."
Translation:Germano vizitas usonanon en Usono.
You could say that if you specifically meant a "female German" and a "female American". As more languages are striving to become gender neutral we understand the Esperanto word "germano" to mean a German, regardless of sex, just as in English. So if you have translated "German" by "germanino" you have added information, and probably got it marked wrong by our computer.
PIV defines -ul- as a suffix signifanta individuon, personon, karakterizitan de tio, kion esprimas la rad.
The "rad" (radiko) here is german- - which has to do with germans and things german. How you word this in English doesn't really matter, since it's not a word you'd ever really see.
To take a different example, you can take azeno and get azenulo which means a person who is in some way like a donkey. (i.e. stupid, possibly stubborn.)
No -ec- needed.