How Early to Arrive for Paris - Luxembourg Train?

Crossing to another country has nothing to do with it, since both France and Luxembourg are in Schengen, there is no border check.
You need to be on board 2 minutes before departure. If you arrive at the station 20 minutes before departure, that is when the Track # will be posted on the departure board and you can proceed to the track and board the train.

You won't know when you leave France and enter Luxembourg unless you're looking out the window and see a sign that says Welcome to Luxembourg.

Other than that, I agree with what Sam wrote.

We arrive half an hour early, so we can orient to the station plan and then find what platform we need and find our train.

For the Eurostar you need a minimum of an hour, but for trains within Schengen as long as you are on the train 2 minutes before departure you are good.

Thank you for the helpful replies! I used to live and work in Paris but it was a long time ago and 5 years since I was there last and everything changes! Even when I go back to my home country of England so much about public transport works differently.

I'll probably get to the station more than 30 mins in advance anyway 'cos it's an early morning train and I'd rather allow plenty of time for traffic and avoid any stress, but it's good to know that I hallucinated the Hour In Advance instruction!

Thanks again!