Budapest

Euros work in a pinch, but get some HUF from an ATM. Credit card worked just fine buying transit tickets out of the vending machines and most restaurants.
Yes, people are helpful and English is widely spoken, especially if a financial transaction is involved. There can be some big-city attitude, same as NYC, London, Paris, Berlin. Pay it no mind. People kept offering this gray-beard their seat on the tram.

When you arrive at Budapest Keleti station, walk to the very front, inside the station. There is an OTP ATM to get Hungarian Forints. I found that credit cards were accepted just as I would expect at home - for my hotels (confirm with yours) restaurants and merchants - and cash for small kiosks and small purchases from vendors at the market. One thing that was different was tips - where credit card was fine for some and cash preferred by others.

The locals in the train station were very friendly in helping me to find the metro.....though a bit shy in admitting they understood English.

If you're taking public transport to get to your hotel, the very large set of stairs (which is behind you as you're getting cash from the ATMs) leads down to the metro and credit-card accepting kiosks where you can purchase a travel pass or individual tickets.

I have nothing to add to the other posts except to highlight the text below from seat61.com: "Seat reservation is optional in 1st or 2nd class for a small extra charge, around €3.50 per seat. If you choose not to add a seat reservation to your booking you simply sit in any unreserved empty seat, but reserving a seat is a good idea in summer or on Friday & Sunday afternoons or at any time if you are a group of several people who want to be sure of seats together." I somehow did not see this when I traveled from Vienna to Budapest in 2015 and did not make seat reservations and we had no seats. It was a very bad situation for a family of four, including a then 9-year-old.

MONEY
The forint is the official currency. Near the river, especially, you will see prices quoted in Forints and Euro. Know, that they dont give very good exchange rates so things will be cheaper in Forints. You can get Forints from any ATM or Exchange House. I prefer the ATM. Taxis will also take credit cards, forints or euros. Most every place you are likely to come in contact with will take your credit card. But always check before assuming and carry a little emergency cash.

I've done the Vienna to Budapest trip twice, once in 2nd class and the other 1st (I'd recently had major surgery and wanted a bigger, quiet compartment and ease of luggage retrieval) and it's best to reserve a seat either class. Of course you should use the forint in Hungary, as others mention there is an ATM at the train station that worked fine for me.

I'm in the "locals are friendly, wonderfully helpful people" camp, though I am working on learning the language and they always seem pleased and amused to hear my feeble attempts--in Budapest you'll be fine with English, though, in the tourist areas.

I read somewhere that Hungary has the greatest number of English speakers for a non English speaking nation. We never had any trouble finding someone who spoke English.

Mike Tipton:

I think this is the basis of what you read: https://dailynewshungary.com/hungary-is-among-the-top-english-speaking-countries/

The article says nearly 60% of Hungarians speak English. I would suspect that the numbers are slanted towards those living in major cities and with about 15% of the population living in the Budapest metroplex I would suspect that the percentage speaking English in Budapest is much higher. Then, if you consider that most of the people you will come in contact with are under 45, and that the group that is under 45 probably makes up the higher percentage speaking English .... Functionally, you will have no problem navigating Budapest in English. I haven't had a problem in the nearly 20 years I have been traveling there. Well ....... 20 years ago t was a bit tougher, but the people are good and patient so it still wasn't a problem.

I always tell my friends and guests, if the person you are speaking to doesn't speak English, then the person standing next to him does. And, not to overly generalized, but more often than not, while they get a kick out of you trying to pronounce Hungarian, they also take a lot of pride in showing you their English so it works both ways.

A worst case language barrier scenario.

Many years ago I went to Buda to do some shopping. I had a dentist appointment sort of on the outskirts of Buda that afternoon and my plan was to call a taxi when I finished the shopping and go direct to the dentist.

So from the corner of Csalogány utca and Gyorskocsi utca (or someplace with equally impossible to pronounce names) I called CityTaxi. The dispatcher asked where I was. I looked at the street signs and just knew I was screwed. "Never mind, I will find someone who speaks Hungarian and call you back" was all I could say.

But the dispatcher didn't want to give up so quickly. I guess we spent 3 or 4 minutes trying to understand spelling and bad pronunciation (I've take enough Hungarian lessons where, in my mind, I can read the word and pronounce it - even if I have no idea what it means). He just wouldn't quit or stop laughing. He finally found me by having me give him the names of the businesses nearby.

Now in similar situations, I will go to a shop or restaurant, or even someone standing next to me on the street and ask politely in English, as I hand them my phone: "could you help me and tell the Taxi company where I am". Never received anything less than a smile and "sure". Once, I got a, "too expensive, come with me" and two buses later I had been escorted home by a sweet old lady.

Very interesting debate about English speakers in different countries. We who travel know that the highest number of English speakers are in Netherland, Scandinavian countries, Switzerland. There you can start speaking in English without asking politely: Do you speak English? I don't think it is possible to measure how many English speakers are in Hungary, Czech Rep., Poland, etc. Because there are different levels of English fluency and each speaker is evaluating themselves. I noticed for example that people in Czech Rep. even if they speak passable English would say they don't speak English, on the other hand people in Thailand or Nepal say they speak English and in reality they know just few words.

Ilja, the debate over who has the most really isnt relative in my world. The debate would be do enough speak English were there are no serious impediments to getting around. I'm not sure if I would like a place as much if everyone spoke English. It would take some the edge off the trip. On the other hand, Moscow was a tad difficult because there was so little English. I even find enough English in Ukraine to get around with little problem. And when there are language gaps, a map, a smile, shrugging the shoulders, and a few hand gestures can be a bunch of fun. People worry too much about language.