What would give you the impression that anyone was taught to use a sword in their left hand? For most medieval people (and probably most people through human history), formal weapons training would be something limited to the elite, who had the available leisure time and wealth necessary to spend their time training to fight all day. Not to say that ordinary people never practiced with their weapons (in fact there is quite a lot of evidence for ordinary people owning and wielding weapons), but it was less like taking lessons from a fencing-master and more like practice fights with your friends and neighbors (or real fights with your enemies/neighbors). This doesn't mean that it's impossible that anyone ever learned how to fight with their left hand, but there is no evidence I am aware of that being taught to fight left-handed was something commonly done.

The overwhelming majority of fencing treatises show right-handed fencers. There are very few illustrations or mentions there of using the left hand, and in most cases it might be for the sake of better illustrations or just mistakes. One of the masters of the Bologna tradition - I believe Marozzo or Manciolino (first half of 16th century), mentions that before learning to fence with a case (that is two) swords, one must learn to use them equally well with each hand. But this is more in the context of "play" fencing made to show skill and look fEncy (Im sorry, please dont ban me) as opposed to street fighting.

The above-mentioned Manciolino also gives some advice for fighting against lefties:

During combat with a left-handed person, stepping continuously against his sword is the optimal protection; and when he throws a riverso, throwing a mandritto to his sword hand, or when he throws a mandritto, throwing a riverso to his hand or to his sword arm, it is not to be doubted that victory is assured.

Source of the quote: "Opera Nova" - Antonio Manciolino, translated by W. Jherek Swanger and published on Wiktenauer.org

In more normal terms - Move towards the side of the opponents sword, when he tries to hit you from the left hit them in the hand from the right, or vice versa. No doubt the same tactic would work against the right handed person, executed by the leftie ( and it doesn't count as a source, but can attest to it from personal experience in training HEMA).

Where did you get the idea that swordsmen were taught to wield a sword in their left hand?

I mean in Europe, any time from Medieval to 1700'd