Wouldn't this translate to,"The boys are Roman"?
Where's the "in" part? I guess it's apart of Romae
If you want them to be "Roman" you would instead use the adjective "Romanus, Romana, Romanum."
Pueri Romani sunt.
The boys are Roman = Pueri Romani sunt
It's written "Romae" because cities, towns, and small islands take the locative case, which happens to take the same endings as the genitive case. So you don't need to use "in", because the ending (and the context too) shows it's a location.
I'm still a beginner but my understanding is that "Romae" is the locative form of "Roma". So "Roma" means "Rome", and "Romae" means "in Rome". A similar thing happens with "urbus" (city) and "urbe" (in a/the city)
The stem of "Roma, Romae, f." is "Rom-"
The feminine locative ending isn't the -e but the -ae, which is identical to the genitive case ending.
Here yes (ae), but you can't generalize it.
The "e" doesn't always work as the locative. It's a wrong way to consider it.
Instead, you have to consider that a word with a gender, and a ending, has always the same declination for a specific case.
You couldn't add "e" to any gender and ending, and make it a locative.
For instance, the locative to say we are in a city:
Romae sum (I'm in Rome, Italy)
Lugduni sum (>I'm in Lyon, France)
Lugdunum is the nominative.