Hmmm ... if you're going to use that line to denote national boundaries, you're going to have to mark it separately, since it's not a conventional way of delineating borders. To my eye it looks like a different kind of vegetation or terrain. You've marked the rest of the "human" geography (names and places) in black - black text, black dots of various sizes for settlements. You should use something visually similar (i.e. black or sharp and obviously unnatural) to define borders. The classical _ . _ . _ pattern is one option. You can then shade the sides of the border or whatnot.

Honestly, I never would have guessed that's a border.