I am a little confused. Isn't "en" translated in English "a" and "ett", "an" in English? Do I have it backwards or is this a spelling error of Duolingo's?
No, they don't translate into each other. You use "an" in english, when the first (spoken) letter of the noun is a vowel. So you can say, it's a phonetic thing.
"En" and "ett" relate to the gender of a thing. "en" words have a grammatical gender and "ett" words are 'neutral'. So its a part of the noun itself. (You just have to learn them.)
jag vilja ett smörgåstårta ^ is that how you say "i want a sandwich cake" in swedish?
Talking about oranges you mean :)?
Well, vilja is the infinitive form so it is "jag vill"
want + noun = "vill ha" + noun
tårta is an en-word
Jag vill ha en smörgåstårta.
"Jag vill ha smörgåstårta" without the article works as well.
"Jag skulle vilja ha (en) smörgåstårta" = I would like (a) sandwich cake
If it should be a common gender and a common gender has a definite "gender" (am I right?) then apelsin should be masculine, am I also right? In Russian, we define a gender of "en apelsin" as masculine. (We Also use the same word for it).