Not everyone chooses America to learn English. I suppose it's more popular because it's more populous, but I know you can get a good result from US, UK, Australia, Canada, NZ, and probably other places where standard English is the dominant or only common language.
You should also keep other things in mind. Have you considered a school, where to stay, what climate and lifestyle you like, the relative value for money in each of the countries?
I can't answer about whether you can achieve a quick education in English. If you're an outgoing person who is willing to speak English to strangers, you should do well anywhere I mentioned.

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