In most major cities around the world, you can get surprisingly far with English alone, at least for transactional needs. Hotels, airports, international restaurant chains, and businesses serving tourists nearly always have someone who speaks it. Younger people and university-educated professionals tend to speak it best, since English is taught widely as a second language.
That said, proficiency varies a lot by country and by situation. Northern Europe, Singapore, and former British colonies sit at one end, while parts of East Asia, Latin America, and continental Europe can be patchier once you leave tourist zones. Taxi drivers, market vendors, and government offices are often the weak spots even in otherwise English-friendly cities.
My practical advice: assume English works for tourism but not for bureaucracy. Bring a translation app with offline packs downloaded, keep your destination address written in the local script, and learn numbers and basic phrases. For anything official like visas or banking, plan for documents in the local language or budget for a translator.