Genesis 11 is going back and detailing a story that clarifies Genesis 10.
Why so many languages?
In Genesis 10:5, 20 and 31 we are told that the descendents of each of Noah's sons moved on, "each with his own language, by their clans, in their nations."
All being from the same family it would seem odd that they each had their own language so quickly.
Genesis 11 reveals how and why this happened.
Also, in v9,10 we are told of Nimrod:
9 He was a mighty hunter before the LORD. Therefore it is said, “Like Nimrod a mighty hunter before the LORD." 10 The beginning of his kingdom was Babel, Erech, Accad, and Calneh, in the land of Shinar. (ESV)
Setting the Scene
Genesis 11 opens by resetting the scene, informing the reader or listener of when and where the narrative is moving to.
1 Now the whole earth had one language and the same words. 2 And as people migrated from the east, they found a plain in the land of Shinar and settled there.(ESV)
If people are just starting to settle in Shinar, then the reader knows this must be back in or before the time of Nimrod, for we just read his kingdom began in the land of Shinar. The apparent contradiction in being told there is one language after being told there were many can be understood as a narrative device, forcing the reader or listener to resolve it. The rest of chapter 11 unfolds and the gaps in the earlier narrative are filled and the contradiction is resolved.
We learn that each of the nations obtained their own language through the Tower of Babel.
It is not unusual for a narrative to give a succinct and possibly incomplete account of an event or period of time only to go back later and fill in the details.
The most obvious is in Genesis chapters 1 and 2:
Genesis 1:27 So God created man in his own image,
in the image of God he created him;
male and female he created them.
Genesis 2:2 And on the seventh day God finished his work that he had done, and he rested on the seventh day from all his work that he had done.
Genesis 2:7 then the LORD God formed the man of dust from the ground and breathed into his nostrils the breath of life, and the man became a living creature.
Either we have to construct an elaborate interpretation where God made man on the sixth day, rested on the seven and then sometime after made another man and named him Adam and placed him in the garden, OR simply understand that this is a detailed description of the creation of man. Genesis 1 is not concerned with these details, it is about the bigger picture, all of creation, but Genesis 2 focuses in on man.