"No country is the most powerful one."
Mmm... maybe no one country. I think you can definitely identify rough groups of states that are at the upper echelon economically, militarily, politically, etc. and look to that group as the most powerful group.
And among them are English-primary states, such as the United States and Britain, and non-English-primary states, such as India and China.
But *all* of them speak English in business. English *is* the trade language, and the closest thing to a common language among the most powerful bloc of states in the world. Consequently, because these most powerful states use English in business and trade, less powerful states that do not share a common language may use English to trade. And so on and so forth; such that, while English isn't a universal language, not by any means, it is the closest thing to a common language in the world.
And, therefore, is the default assumption on websites like this one. It's not mandatory. It's a guideline. If, for example, you were to negotiate with me in a game in Turkish, I would take the time to use Google Translator and figure out what you're saying (and I would respond in Turkish using Google Translator), but I would certainly ask if you speak English fluently enough for practical negotiation, and if the answer were no, then I'd be very inclined to negotiate more extensively and favorably with someone who speaks English fluently enough for practical negotiation. (For sake of completion, that wouldn't happen; your English is fine. Just using an example.)
It's a practical convention, not a racist, supremacist mandate.