Perhaps you should take a look at a brief history of the English language so that you can understand that there really is no one "proper, fluent English".
https://www.oxfordinternationalenglish.com/a-brief-history-of-the-english-language/
That's the thing.
It's hard to find an English people who speak with a "standard pronunciation". A standard language without influenced by local/ancient dialects.
For Arabic, the standard of Arabic can be found in world's champions of Quran recitation competitions. Probably the champions so far are mostly nonArabs. So called "the world of Arab" actually came from different ethnic/language speakers: Coptic, Aramaic, Berber, Babylon etc.
For Indonesian language, the standard can be found mostly in news broadcasters coming from the capital city of Jakarta. They belong to various ethnics. Jakarta gathers them into the same standard of Indonesian, even though Indonesian language by origin comes from Melayu (Riau archipelago)
As to English standard, it is said, there has been "born" a new accent of English by millennial generation from various nations. They speak the same accent which is not found anywhere before.
I still haven't found what kind of accent do they speak? Any link please?