The main reason why the author mentions the diffusion of English was to В. offer evidence in support of his basic thesis.
This refers to the main reason why a text is written by an author to convey an idea to his audience.
Hence, we can note that from the complete text there is the narration of how the diffusion of English has a certain effect as it is "almost global" and this has an opposite effect so as to offer evidence in support of his basic thesis.
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