Genesis 11:1 says that the world at that time “had one language and a
common speech.” However, Genesis 10:5 seems to imply that there was more
than one language at that same time. Please explain this.

I can think of a couple of possible explanations, but the most
obvious one is probably correct. Although Genesis 10:5 comes before
Genesis 11:1 in the Bible, it is referring to a later time. The time
frame which is represented by the geneology; Japheth-Javan-Elishah-etc…
may be hundreds of years. It was very common for Jewish geneoligies to
skip one or even several generations. Therefore, by the time Japheth’s
descendents had multiplied into many nations, such as the Rodanim, the
incidents at the tower of Babel may have been ancient history. The
context of Genesis 11:1-9 seems to be a few generations after Noah and his
sons, but Genesis 10:5 appears to reach to many generations–well past the
Babel events. Because neither account gives a definite number of years,
it is hard to prove either way, but this appears to be the natural reading
to me.

John Oakes