It can't be because of different languages and accents, could it?

It's more likely to be different cultures, behaviours, or religions.

As an example, many Jewish individuals maintain tight-knit communities, and do business with each other. They have a strong focus on education, a hard work ethic, and an median IQ higher than most other populations. They have been the victim of a lot of persecution over the years (which tends to bring groups together), and (like many ethnic groups) tend to go into certain industries. Because they have connections in those industries, it's easier for young adults to go into those industries, because they are more likely to have access to job opportunities, and experience in those fields).

So, the economy starts to do poorly (as was the case in Germany), and huge reparations are required. The wealthy were more likely to be Jewish, which put them in a position to loan out money - something that breeds a lot of resentment. Inflation gets to be bad, and that disproportionately hurts wage earners compared to investors and people with non-monetary assets.

Now, we have the Hans Schmidts of Germanly looking around at businesses closing down, and getting bought - by rich Jews. They see the Rothschilds and the like getting even richer by taking advantage of cheap opportunities and high interest rates, and they start looking for someone to blame.

So, you have a largely white group getting mad at another largely white group, because they are living different lives, and one blames the other for their suffering. Meanwhile, you have Jewish populations opposing any kind of ethnic unity and promoting multiculturalism because a tolerant society is safer for them.

Skin is often just a convenient proxy, with sometimes hilarious results. I was born in Japan. My brother is ethnically Korean. In Japan, I would be tolerated, and he would be looked down upon because the Japanese are very racist.

In America, the majority lumps all the Asians together, and so in his high school, they all stick together. Because they are a minority, they can't really build separate Japanese, Korean, and Chinese groups, so they band together as the Asians. In Asia, they would be completely separate groups.