I'm not a linguist and I find the whole excercise of conlanging, while I love it and respect it, beyond my abilities, but I do have one thing or two to say about linguistic diversity and how boring is to have a "common" or "basic" language in fantasy or science fiction without exploring the implications.
Being a bilingual speaker of Spanish and English, and someone that because of work reasons and entertaiment tastes interacts a lot with English, I tend to see English as the equivalent of those "common" or "basic" languages of speculative fantasy. As a useful tool for communication, science technology and commerce. In real life, however, as you are aware, the expansion of English tends to undermine local languages, it's considered more valuable to know English that to know the language of your grandparents, or learn any other language you just feel curious about.
The experiences of every multilingual person are different, but in mine I know English, I write and read and listen to English a lot. But I don't consider myself an English *speaker*, I speak Spanish and more to the point Argentine Spanish, that's the culture I identify with, and it's the language I use to express my feelings and inner thoughts. I can't imagine saying "I love you" to anyone in English, to me it's just a tool I use to access to knowledge or communicate through language barriers ("basic", "common"). But interestingly, by both writing and participating in the wider English-speaker internet culture, isn't it part of my own culture, as an individual, too?
The fact is that English also has a culture(s) and a history and a corpus of literature. So when we write about "Common" or "Basic" languages in fiction we need to ask ourselves: where did they come from? How did they become the standard? Is there a literature, a canon, a culture of "Common" in your fantasy world? What about other languages, other cultures that aren't raised learning it and see it just as a tool? Because no matter the strenght of Anglophone cultural imperialism and the social value of learning English, I don't see Argentines, or for that matter Chinese, Italians or Russians abandoning their first language. And yet even in English and in all other languages (ESPECIALLY other languages, English is remarkably uniform) there is a variety of dialects. And we need to remember, once Latin was spoken only in a village in central Italy, and English in a rather remote rainy island. They weren't destined to have their future roles, history drives language.
So, when an author goes for the "universal language" explanation to avoid linguistic misunderstandings, for me, it raises more questions that I believe are worth exploring.
