I have never been to China. I'm from the US and I only speak English. Currently I'm living in the Philippines though.

I imagine China is a lot like the Philippines. There are a lot of dialects and then one national language. The national language here is Tagalog, but English is also a national language.

You can expect that almost everyone speaks the local dialect unless they are from another region. You can expect that most people speak conversational English. Some have a difficult time speaking Tagalog and may revert to English if Tagalog is the only common language among the speakers.

People feel confident speaking the languages they use regularly. For regions outside the Tagalog regions, that's mostly the regional dialect. Most street signs are in English. Local TV is mostly Tagalog but there are English stations as well.

Everyone is unique, but in many cases people have no reason to speak Tagalog. They may hear it, but hearing is different than speaking. In any case, you can't make the same assumptions nation-wide as you can with people in the US.

In the Philippines, you could do pretty good with marketing in English. Kids are taught English in school and it's possible the regional dialects could quickly disappear in the future. If you're going to learn only one language in the Philippines, it should be English. Other countries may not have this same unity on language. Each country is different.

In the US, you have (Wikipedia 2019 numbers) nearly 330 million people who you can make solid assumptions for in terms of language. Most of those people are English as a daily driver if not a first language. It's hard to beat the combination of one of the most widely spoken languages and one of the largest countries by population.

I don't know what other language has that many "first language" speakers in a single country. Mandarin could be, but even within the Mandarin language there are lots of dialects (one caveat is that the writing system is the same for all.) So, it's difficult to determine what the actual numbers are. According to a quick Google search, a quarter of the Chinese population doesn't speak the common tongue at all.

An honorable mention might be Brazil, a country with a population of over 200 million and all-in on Portuguese.

While the Chinese government has forced the majority of young people to learn Mandarin in schools, even just taking a 1 hour train ride from Shanghai to Hangzhou, both cities have local languages that are mutually unintelligible. Elderly people, especially in the countryside but also in the cities occasionally, often don't know how to speak Mandarin at all. But if you mostly hang out with urban youth you should be fine entirely with Mandarin.

Unless you only speak to younger people in tier-1 cities, China is enormously diverse linguistically.