I'm gonna give you an answer to what I think you're asking, but in case I've misread it, could you please clarify? There are a couple questions you could be asking. Do you mean "land border" as in political borders? Or just things like geographic boundaries? Or do you mean do languages have boundaries unrelated to those things? I'll try to give you a more broad answer that touches on these, though I think you're asking the last one.

Language diversity tends to follow geographic boundaries. Things like rivers, mountains, deserts and other things that are difficult to cross have always been significant points of contrast between related languages/dialects, as well as the boundaries between unrelated neighbouring languages.

Trade routes affect transmission of language and diversification of dialects. Things that get in the way of that create stronger contrasts. In my own research there's a group of very closely related dialects, and in almost every case where going a short distance from one place to another where there's a clear difference (rather than a gradual one), it's because of a geographic feature or, less often, the result of an abrupt migration/resettlement of/by a group not previously in that location.

Do related language varieties have clear boundaries otherwise? No, at least not if we're looking at the big picture. Instead what you have is a bunch of different boundaries that are limited to one or a few features. Something like this:

http://csumc.wisc.edu/AmericanLanguages/graphics/europeanrootswisger.jpg

What you're seeing there is dialects of German. The green/yellow/orange are marking dialect groups (West Middle German, East Middle German, Alemannisch etc). But the important thing in this case is the red lines. Those are marking isoglosses. These are things where a given word (labeled w/ the lines) is pronounced or used differently on the two sides of the line. Here's a similar map with only one line:

https://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Ik-ich-Isogloss_-Uerdinger_Lien.svg/2000px-Ik-ich-Isogloss-_Uerdinger_Lien.svg.png

What that shows you is that north of that red line, ich is pronounced "ik", and south of the line it's "ich". That's what the first map is showing, but with a much greater number of isoglosses.

Features will have boundaries like this, and different varieties will have one or the other, and it's possible that there's an "ich" island north of the ik/ich line (just hypothetical in this case), but there is generally not one line that covers a wide range of features where there isn't otherwise some geographic or political boundary, and even then the isoglosses might not line up that well.

I hope that answers your question. If not, or if you have follow ups, let me know.