In summary, English has become the leading international language due to three different factors. Firstly, the language came to have a strong position in large parts of the world through the empire that Great Britain established from the 17th to the 20th centuries. Secondly, the USA became a leading nation technologically, economically and politically during the first part of the 20th century and continues to hold this position. Thirdly, industry, communications and international contacts have developed in the 20th century in such a way that a common language for contact across language regions has become much more desirable and necessary than before. English was there to fill that need, while the other European languages for various reasons were pushed aside. (Source: "The languages and the history", Tore Jansson)
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That is a good research question. To our knowledge, it has never been studied by contrasting these two questions in comparable surveys. In arguments you have to persuade your opponent in some way, and persuasion, like all parts of rhetoric, demands full control of the language that you have to use in that situation. A politician who is forced to use a language that he/she can't use at an adequate level acts against his/her own interests (and those of his/her voters). The same applies to ordinary citizens.
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