I did the journey the other way a couple of years ago. Brasov to Bucharest. I chose a train that was described as fast. It tuned out to be a loco hauled service from Austria (or further afield). Very comfortable. Took about 2 hours I think. We went 1st class which was only about £5 extra. 1st class turned out to be adjacent to the Buffet/Dining car. Handy enough. We got chicken and chips and some beers for a reasonable price.
Scenery very impressive around Brasov but you gradually descend to a plain as you approach Bucharest. So if you are going the other way it should get more impressive as you climb into the hills.
Some of the train staff will speak a little English. And Romanian is pretty similar to Italian so if anyone in your party speaks Italian, you can bluff with that. Otherwise I suggest you go to Google translate and get it to translate what you want. Eg “A return ticket to Brasov, 1st class, returning on X date.” Write it out on a piece of paper and hand it to the ticket office staff. They'll usually write the price on the paper and you can proceed from there. Part payment with Tesco vouchers may be a bit more complicated but for a simple transaction it worked fine for us.
Watch out for the taxis at Bucharest station which will rip you off (in my experience). They’ll try and charge 15 euros (not Lei) for a trip round the corner. But you may not need them, and there is an underground service.
My experience of Romania generally was of a very friendly, honest and slightly laid back country. Europe in the 1940s. People still using horses and carts. Local trains a bit dilapidated (glass in the windows sometimes optional) but reliable enough. Food very good and cheap. Wine and beer excellent. People really friendly and helpful. Strongly recommend the country.
Watch out for Dracula in Brasov, obviously. Don’t go out after dark if there's a full moon.