I don't think this instance is much like the difficulty that has been encountered with the costal objectives. But I think the contrast can inform here. Since you brought it up I looked over all the cards to compare grammar.
War Against the Pirates reads: "...at least two forts and/or cities in coastal provinces (excluding your kingdom)." It pluralizes the statement with 'provinces', obviously meaning multiple. Sea Power uses the same form.
We can assume the game maintains unity of form for intention. If the Border Wars card is intended to refer to multiple provinces, it would use the same grammar as the sea cards. Since it does not ("...a savage province..." rather than " ...savage provinces..."), the only logical conclusion is that it refers to a single savage province.