Why are there so many languages when the origin is one?

Allah knows best. Your Lord is the Most Wise, All-Knowing. We do not have
any certain knowledge of the wisdom behind that, but we know that our Lord
is Most Wise, All-Knowing. He, may He be glorified and exalted, says
(interpretation of the meaning):

“Certainly your Lord is Most Wise, All-Knowing”

[al-An ‘aam 6:83]

“And Allah is Ever All-Knowing, Most Wise”

[an-Nisa’ 4:11]. 

By
His great wisdom, He made many languages and made people of different
colours, as He, may He be glorified and exalted, says (interpretation of the
meaning):

“And among
His Signs is the creation of the heavens and the earth, and the difference
of your languages and colours”

[ar-Room 30:22]. 

The reason may be so as to highlight His almighty power, for He, may He be
glorified, is able to make one language for some people and another language
for others, for this is more indicative of great power. 

There may be other reasons for it of which we are unaware and that we cannot
comprehend, but scholars other than us may understand them. 

The point is that one of the clearest reasons behind it is that He, may He
be glorified and exalted, is able to do all things, therefore He gave people
many languages and told us that this is one of His signs (interpretation of
the meaning): “And
among His Signs is the creation of the heavens and the earth, and the
difference of your languages and colours”
[ar-Room 30:22]. 

Just as He made them of many colours, including red, black and white, and
shades in between, and He has caused them to vary in size, so that some are
tall and some are short, and some are in between, and He has caused you to
vary in attitude and intellect, the same also applies to the issue of
languages. All of that is indicative of His almighty power and highlights
the fact that He does whatever He wills, may He be glorified and exalted,
and there may be other great reasons behind that which we do not understand.
End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).