Learn Croatian: Jeste li vi iz strane zemlje? - Are you from a foreign country?

This happens because of the preposition iz (from), which requires the genitive case. The noun zemlja (country) is feminine, so its genitive singular ending is -e. Because adjectives must match their nouns, the adjective strana (foreign) also changes its ending to become strane.

I thought "zemlja" meant "earth" or "soil". Can it mean "country" too?

Yes, zemlja is a very versatile word! Depending on the context, it can mean "earth", "soil", "dirt", or "country". In this sentence, strane zemlje clearly means "foreign country".

Why do we use "jeste li" instead of "ste li" for "are you"?

When forming a yes/no question with the little question word li, you must use the long, full form of the verb "to be". The short, everyday form ste cannot stand next to li in this way, so we have to use the long form jeste.

Could "vi" in this sentence refer to just one person?

Absolutely! In Croatian, vi is the plural "you", but it is also used as the polite, formal "you" when speaking to a single adult you don't know well. You could use this exact sentence to ask a single stranger if they are from a foreign country.