Translation:The boy has the thirteenth sandwich.
I think it may be the final "a" that makes it a the. So it is likely feminine like carte becomes cartea maybe?
There seems to be a variation in the spelling of 'sandwich' in Romanian? I have seen it spelt 'sandvici' (on Duolingo) and 'sandviș' (elsewhere). Is there a regional variation?
The DEX (romanian dictionary) has sandvici, sandviș, sanviș and sendviș. The other ones are incorect. https://dexonline.ro/articol/Forme_greșite_ale_cuvintelor
Nah, it's has different variations bacause it's not a romanian word and it does not have a synonym in the romanian language. The correct versions are (following the book DOOM: Dicționarul ortografic, ortoepic și morfologic al limbii române): sandvici and sendviș... this is from 2014 through and some rules might have changed with the appearance of a newer version of the dictionary.