There is a friend of mine who constantly makes racism tests on me. I’ll now explain to you what this “racism test” is. Basically, he makes racist remarks and tries to understand how I’ll react to them. And then he makes conclusions from my reactions. It’s very annoying and I do not know how I can stop him from doing that to me. I politely asked him not to do so, but that was no use. I threatened him with violence. It was no use either. I slapped her nape, which was the last level of harmless violence that I could utilize to signal him that he was going way too far with his remarks and his unintelligent test. This was not helpful as well. Beyond this harmless level of violence lies a strong punch highly likely to leave a long-last lasting bruise on his pretty face. Yet, I do not want this. I want him to understand the depth of his error. I want him to not do it again, not only to me, to anyone else. And, in case he ever happens to do, people should call me into action. Why me? The answer is simple, unfortunately I am the only one who knows the “language” that he understand.
Your reaction is that you still hang out with this clueless asshole, and we can all draw conclusions from that.
If he’s too self-involved to care about the effects of his repugnant language on you, he’s not your friend. Get him out of your life; find some better people.
Completely ignore him. See how he reacts to that.
Dump him. He is not a friend.
This, or the more dramatic way of making the same point – say nothing, turn around and walk away from him.
I’m unclear from your OP whether he is using racial slurs in front of you, or directing racial slurs at you.
Either way he is totally fucking with you, and is probably unbalanced. Dump this shit before he decides to do a loaded gun test on you.
Are these racial slurs that your friend made up that he has made you aware of?
Are you a translator for him in some official capacity?
Is there a reason you can’t ignore him when he starts his ‘tests’?
What language is this that no one else can understand? If no one else can understand it then who cares?
The OP is somewhat confusing but I don’t think he’s talking about a literal language. I think he means it metaphorically as in “I had to talk to him in the only language he understands” and he appears to be referring to punching his friend in the face if he won’t stop.
My advice to the OP is to stop hanging out with this guy. He shouldn’t be playing head games with you and you shouldn’t be thinking about punching him in the face.
Tell him
“Good day, Sir. I said, ‘GOOD DAY!’”
People who intentionally push your buttons or attempt to “get inside your head” are not to be trusted. Walk away.
Have you considered that your friend is not testing you, but is simply a racist asshole saying what he believes?
You know, we ban people for trolling around here. It’s not outside the bounds of reason to apply this rule to your personal life as well.
God yes, this. If anything, I’m quicker to cut people out of my real-world circles who engage in trollish behavior.
N/M
One technique that might work is to ask him to explain his remarks in detail:
“What do you mean by that?”
“What makes you think that?”
“What sort of evidence do you have to support that?”
Asking lots of questions is less work than answering lots of questions. And most trolls don’t like answering lots of questions. It’s boring and puts them in the uncomfortable position of having to explain something they may not believe.
Or just dump him.
I agree. I’ve come to realize that friendship isn’t some sort of sacrosanct institution that must be preserved at any cost.
If you get a girlfriend, you’ll never have to hang out with this loser again. 
On the subject of language— out of curiosity, did you compose your OP in a language other than English, and then use Google Translate (or something similar)? I notice some peculiarities in your post that suggest this.
I was wondering about the OP. MaverocK seems to be talking about a male friend and then says, “I slapped her nape” whatever that means.