"A cook speaks Swedish."
Translation:En kock talar svenska.
Which words come with "en" and which with "ett" is one of those things you just have to memorize. En-words are more frequent, and as far as I can tell most words that refer to humans are en-words. But ultimately, it's best to just memorize each new noun with its appropriate article - for example "en bok/boken", "en kock/kocken", "ett glas/glaset" "ett äpple/äpplet", etc.
Yea, it comes from masculinity and femininity stuff similar to romance languages. There used to be 3 categories: masculine, feminine and neutral but they combined the first two to make the langasge easier, which is why most words are en and the rest, gender neutrals are ett. There is no clear telling so you need to learn nouns with their type but most of the time assuming en is helpful
Svenske is masculine singular. Den svenske mannen (masculine) Den svenska Kvinnan (feminine) Det svenska barnet (neutral) De svenska männen. (plural) Now if use neutral instead of masculine, or masculine instead of feminine, most people won't care, and the ones who do will simply think it sounds a bit strange, so there's no need to worry about it.