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Welcome to International Site Expansion! This is the exact situation I worry about when people set up international structure. The answer lies in what your users need, not what you want or what marketing wants. Do users in the US need to see different content than a user in the UK or Australia? All of those countries have English as the primary language. I am not talking about dialects (UK spellings/words vs US words), but do the users need to see different content due to product set changes, price changes, different marketing tactics, etc.? There are a whole list of questions you need to go through. http://outspokenmedia.com/international-seo-strategy/ - Let me know what your end result is and we'll talk from there. Another resource: https://moz.com/blog/guide-to-international-seo (that will explain a lot)