Learn Russian: Южане никогда не видели снег.

Nouns ending in -анин or -янин that denote nationalities, regions, or social classes drop the -ин part in the plural. Then, instead of the standard -ы or -и ending, they take an -е in the nominative plural, making it южане.

Why do we use the imperfective verb видели here for "have never seen"?

In Russian, when you talk about general life experiences, such as whether someone has ever done something or never done something, you almost always use the imperfective aspect. Since they have never seen snow in their entire lives, there is no completed, one-time result to focus on, so we use видели instead of the perfective увидели.

Does никогда не видели literally translate to "never did not see"?

Yes, Russian requires a double negative. Whenever you use a negative word like никогда (never) or никто (nobody), you must also place не directly in front of the verb. Unlike in English, this does not cancel out the negative meaning; it simply completes the grammar structure.

Since the sentence is negative, shouldn't снег be in the genitive case as снега?