Learn Russian: Северяне любят сильный мороз и снег.

Nouns for nationalities or groups ending in -анин or -янин in the singular drop the -ин when forming the plural. Instead of adding a standard plural -ы or -и, they add -е. Therefore, северянин becomes северяне.

What would the accusative plural of северяне look like if they were the object of the sentence?

Because северяне refers to living people, it is animate. For animate nouns, the accusative plural matches the genitive plural. Nouns ending in -яне drop the final -е to form the genitive/accusative plural, giving us северян. So if you wanted to say 'I love northerners', it would be Я люблю северян.

Why do сильный мороз and снег look like they are in the nominative case here?

They are actually in the accusative case because they are the direct objects of the verb любят (love). However, because мороз (frost) and снег (snow) are masculine and inanimate (non-living), their accusative forms look exactly the same as their nominative forms. The adjective сильный (strong) matches this pattern too.

Is there a different word to use if all the northerners are women?