Yes, “La Marseillaise” has been used by the left for their own purposes. But I’ve read the lyrics in English. It talks about liberty, and ridding France of kings, mercenaries, and foreign troops. But that’s it. I see no commie stuff.
Of course your mileage may vary.
The entire French revolution was a phenomenon of the Left. It’s where we get the terminology of Left/Right. The Jacobins were leftists. I’m sorry for you to find out this way, but to be anti-monarchist is to be on the Left, by definition.