Basic story, Conquerors enter a land and take it over. They set up their own systems. The Conquered may be excluded. Some might wind up being taken into the system.
The language of the Conquerors becomes the standard language, the education and culture become the standard, and the Conquered either absorb the Conqueror's standards, or don't.
Those of the Conquered who can speak the language and understand the culture will be moved up in society. Those that don't, fall into the underclass.
In many cases, the conquered territory becomes a colony of the Ruling Power. As a colony, those who comprehend the Ruling Culture move up, those who don't stay down.
As generations pass there comes a point in which the Conquered are just as qualified as the Conquerors and often, Colonial powers fade and the territory is left in the hands of the Conquered.
Here is where the dichotomy begins.
If one speak the language of the Colonial power well, one can proceed upwards in society. This allows members of the underclass to become the overclass.
This causes a great discomfort for the ruling class and to attempt to erect barriers to prevent this will result in civil war. However, if one can encourage members of the underclass to cling to their 'culture' or their 'language' this would effectively disqualify them from becoming members of the Ruling Class.
Years ago, many colonies gained Independence.
When they did, (focusing on just one 'Empire) they spoke English, attended schools which were on par with those in England, and had a culture which was not that diverse from their ex-Colonial Masters.
As time has passed and the push to maintain 'class' barriers has increased, one will find that many people in diverse territories have opted to not speak English, not ascribe to English culture. In this way their segregation is self-chosen.
It is not that a child is denied entry into University because of race, religion, ethnicity, it is that the child can not read, write, and speak English so fails the entrance exams.
The applicant is not rejected because of race, religion, ethnicity, it is because of the inability to speak English.
It is not that the 1st World discriminates against the 3rd World because of race, religion, ethnicity, it is simply that those who can gain professional requirements are not hired. They can not gain the requirements because they can not speak English. They can not speak English because their local leaders encourage them to speak their 'native language'.
This self disqualification is taking place in all ex-colonies. Draped with a sense of 'independence' and 'pride' they condemn themselves to the underclass.