Plural or singular: "Long live Europe" or "Long lives Europe"
It's not a matter of singular or plural it's about the mood; "long live [whatever]" expresses a desire for the future, "long lives [whatever]" would be a statement of the present state of affairs, if anyone said it that way. The origin of the phrase "long live Europe" is probably le roi est mort, vive le roi or the king is dead, long live the king, the first king being the dead Charles VI and the second his son who became Charles VII from that moment on but the sentiment is probably much older.