And if France had the foresight to build the Maginot Line then why was their northern borders left undefended considering what happened in WW1?

Basically it wasn't. Part of the reason to build the maginot line was to force Germany to make an invasion through the low countries and french military planners were very well aware that a german invasion would most likely come from there. They also made plans for a defense of the low countries that was pretty sound. Sadly they misjudged the German ability to push through the Ardennes forest.

The line did in fact extend all they way to the coast but the parts north of the Franco-German border were much lighter fortifications and not really a continuous line. Part of the reason for this was that building a line of fortifications like the Maginot line was very expensive, another reason was to not offend neutral Belgium by building what is in essence a giant military installation right on their border.

This is where knowing some geography is helpful. While in both wars, the German attack came through the Low Countries, Belgium, the Netherlands, and Luxembourg are not physically homogenous. Flowing north to the sea, the Meuse river makes a sharp bend right and east in let's say the lower third of Belgium. North of the river, the land is low, flat, and well watered, but south/west of the Meuse, there's the large, rugged Ardennes forest.

In 1914, the German attack came through the North Belgian plains, because it's easy to transport troops and supplies over that easy terrain; the intention was to get behind French forces attacking over the German border. With the French border heavily fortified, and the Ardennes considered prohibitively rugged, the French believe that Northern Belgium is the most likely invasion route. The studies they did prewar suggested it would take about sixty hours for a force to cross the Ardennes, but another week or so for them to bring up heavy artillery strong enough to crack through the fortifications on the French bank of the Meuse river. Their plan is to march into Belgium upon being invited in and set up a defensive line along the Deyle River, which runs North-South, east of Brussels. If they link up with the Dutch Army at Breda, Netherlands, they'll have a continuous line from the North Sea to the Alps, and they'll beat the Germans the same way they did in WWI.

The region they were concerned about was called the Gembloux gap, which was the flat country between the source of the Deyle River and the North bank of the Meuse, since they wouldn't have the advantage of a river in front of them in defending that territory. When the Germans invade Belgium with Army Group B, they're going to fight north of the Meuse, devoting three Panzer divisions to fighting in the Gembloux Gap; the Allies think this is the main effort, and it's a massive tank battle. Almost all of the Allies' most mobile, heavily armed divisions are committed to the fighting north of the Meuse.

What they missed was massive force crossing the Ardennes south of the Meuse; Army Group A, with five panzer divisions and three motorized infantry divisions in Panzergruppe von Kleist, made it to east bank of the river in 58 hours, but instead of waiting to bring up heavy artillery to cover their river crossing, the Germans attacked immediately, using their dive bombers as flying artillery against the two low quality divisions posted to oppose them. Even after the air bombardment, though, the Germans were stopped cold at all but two of their crossing points. Still, though, their infantry and combat engineers manage to fight their way forward and secure a crossing for the Grossdeutchland Panzergrenadier regiment, commanded by Hermann Balck. His men take the high ground, and the French position unravels.

Since the attack landed at the boundary between the French 9th Army and 4th Army, 9th sees itself getting outflanked to the south, and retreats north. 4th sees its north flank getting turned, so it retreats south. This opened a gap between the two armies that the Germans poured four field army equivalents through; with all their reserves trying to not fall victim to the Schlieffen plan by defending northern Belgium, the Allies don't have anything ready to stop the German drive to the English Channel, which cut their lines of supply with the French interior. With 1.7 million soldiers cut off and 1.2 million prisoners taken in the Low Countries, it was the greatest encirclement battle of all time.