What are possible legal repercussions for someone who stands up in a restaurant and says something like "this n-r must leave the place where white people eat"? What laws apply here?
In England, this action* would be a breach of the Criminal Justice and Public Order Act (1994), specifically that...
A person is guilty of an offence if, with intent to cause a person harassment, alarm or distress, he ... uses threatening, abusive or insulting words or behaviour ... thereby causing that or another person harassment, alarm or distress.
As to punishment, this would be determined by a judge based on a number of factors, but not to exceed
imprisonment for a term not exceeding six months or to a fine not exceeding level 5 on the standard scale [e.g. unlimited] or to both.
*Noting that this is not 'discrimination', but rather 'abusive behaviour'