The promise was given before Israel existed. Remember his father was Issacm whose father was Abraham, whose great garandfather was Shem whose father was Noah. As we know this is easily well before the other cultures you want to throw into the mix
In terms of civilizations, the oldest is Mesopotamia. The second oldest is India (Indus Valley Culture). The third oldest is Egypt.While it is true that most modern humans are descendants of peoples who migrated from Africa and into the middle-east, and then dissipated into various other areas, this was long before any one culture existed (hundreds of thousands of years ago) and much, much, much, before the advent of Semitic culture, let alone the more recent Judaic Semitic culture, which is younger than the oldest Indo-European culture by secular, non-revisionist estimates.Now if we look at Semitic pagan culture (before the Israelites, monotheism, etc), then that is the oldest.
The promise was made to Abraham and Ishmael was one of his sons, there are your Arabs. Lets look at Jacob and Esau, Esau looked like a bonified Irishman. Lets come on down to the sons of Israel/Jacob, Joseph was very fair skinned, much like the Europeans, he fathered Ephram and Mannaseh, whose tribes are believed to have been taken into captivity and settled to the north, pretty much Europe and Russia. Now look back at Ham Shem and Japheth, the three sons of Noah. Ham and his decendents settled to the south, southern arabia and Africa, Shem the middle east and southern asia, and Japheth, europe and russia. Go on down the line a few hundred years later and you have Abraham and Lot, Lot was saved from Sodom and Gamorah, his daugter raped him and produce Gog and Magog, people who settled in Russia. Now we go to Isaac and Ishmael, which we can skip because I addressed this earlier. Now back to Israel/Jacob and the many exiles and being numerously conquered and spread in all directions. Only Judah and Ben with a remnant of Levi were left in Israel at the time of Christ. many of the nationalities looked so similar that sometimes it was hard to tell just by looking at them.
No one knows what they looked liked, or if many of these people even existed.
You just confirmed what I just said, but in case you misunderstood yourself. Take a look at the Americans and the British, very much the same people group at the founding of the Eropean colonies, but after a few hundred years, the languages sound so different, imagine what a 1000 years could do with little or no contact between the 2 nations. This easily explains the difference in the language structures of many nations through out history even if they had a common language to begin with.
They did have a common language, but this was a looooong time ago and before Semitic culture existed.