“I was so nervous. I was sweating. My legs were trembling.” Kevin-Prince Boateng was a long way from the Berlin streets where he grew up; a long way from the football pitch too, although it was a game that brought him to the United Nations in Geneva. It was March 2013. It was also, he says, “the craziest moment of my life” – and his is some life. There’s a lot to discuss as he eases into a yellow chair: from the eight clubs in four countries to playing his brother at the World Cup and meeting Nelson Mandela. Who, incidentally, tried to marry him to his daughter. But first, this.

It’s some time before Boateng heads out past the technicians laying cables and across the grass at the Estadio Gran Canaria where he plays for Las Palmas on the island that’s become home. “You know, I’m always honest,” he says, getting up. That much has become clear over an hour spent talking with sincerity and humour: good times and too-good times; things people don’t see, like loneliness, and things they do; talent and personality, confrontations and convictions, always willing to make a stand. “I’ve never lacked guts, for good or bad.” That much is clear too.

That day in Geneva, even he wasn’t sure. Intelligent, articulate, multilingual, a man who embraces the “beauty” of language and who his older brother describes as a chameleon, adapting to any environment, he had prepared for this – “almost 24 hours a day for five days” – but still: delivering a speech alongside Navi Pillay, the United Nations high commissioner for human rights. “She tapped my leg under the table: ‘Don’t worry, you’ll be fine.’ ‘No, I won’t.’ I saw 60 cameras, red lights, recording. ‘Oh my God’. At first, I stuttered ‘Urrgh’, then got in the flow. When I finished, everyone stood and applauded. They said no one ever got that.”

“It was very emotional but I can’t tell you it was only beautiful. Personally, it was unbelievable to get the chance to say what I feel, what I’d seen, what I’d experienced in my life. But I was there for a very, very negative thing, so I was torn. It was difficult to go and speak.”

There had been no guidance on what to say or how but Boateng, the son of a Ghanaian father and German mother, had been invited to the UN after he walked off during a friendly between Milan and Pro Patria in January 2013 when he, Urby Emanuelson, M’Baye Niang and Sulley Muntari were racially abused. The message was powerful; it may not have been intended, but he had started something.

Boateng gestures towards the crowd after racist chants directed at Milan’s black players from Pro Patria fans. Photograph: Emilio Andreoli/AP

“It was an automatic reaction,” he says. “Thinking about it, maybe I wouldn’t do it again. Maybe I’d speak to the referee or get the stadium announcer to say something. Walking isn’t always the best response: we have a responsibility. But I couldn’t take it anymore, couldn’t keep my emotions inside. I thought: ‘Why do we still have to go through this?’”

Sadly, “still” is the word, four years on. A fortnight ago, Balotelli was subjected to monkey chants in France. “Speaking to the UN, you reach the whole world, but it’s one day; the next it’s forgotten,” Boateng says. His speech drew parallels between racism and Malaria, likening it to a “dangerous and contagious virus” to be confronted and eradicated and warning it would not just go away. It has not – despite Fifa disbanding their anti-racism taskforce in September, declaring the mission accomplished.

Boateng was the taskforce’s first ambassador, preparing a report, helping draw up action plans, but there was no call, no message, not a word. The work is done. “For them, maybe the work’s done but this work can’t be done,” he says. “We’ve seen it with Balotelli at Bastia. He’s a close friend; he said it’s incredible. He doesn’t know what to do. ‘What can I do? Put something on Instagram, Twitter?’ There’s nothing else. He can’t fight racism alone.”

We should fight harder, stronger, against racism. We can’t say the work is done

“We should fight harder, stronger. We can’t say the work is done. Sepp Blatter and Fifa wanted to change something. Pep Guardiola, Rio Ferdinand, Ronaldo, everybody got involved; they all got my back. We had to do something. Racism goes beyond football, but football gives you the platform [but] then they close [the taskforce] for I-don’t-know-what reason. All I know is I read it one day and I was shocked.”

“‘We achieved what we wanted to achieve,’” Boateng scoffs. “I don’t know what that is, but we definitely didn’t fight racism the way I thought. We had lots of ideas but it didn’t change nothing. Just saying no to racism on a commercial doesn’t do it: it’s nice, but you only see it during the Champions League …”

There is a rueful smile. “And now they took me out. I suppose because I’m at Las Palmas now, not playing the Champions League, but this is a message about racism, not the Champions League. Of course you get big players like Ibrahimović, Neymar, Messi, Ronaldo because they have visibility but what did they do against racism? It’s funny – the only player to give a speech at the UN and they take me out.”

‘If you don’t die, I die’

Boateng was raised in Wedding, a “rough” neighbourhood of high unemployment and high immigration where he says “people were shot and police didn’t even enter”, where he recalls Koloniestrasse, the street next to his, being in the “top 10” most dangerous, and where the “rules” were “if you don’t die, I die”. It was the kind of place that shaped you, he says. “We didn’t have much money but that was my life. I didn’t know better so it was OK – and the things you learn and see on the street make you who you are.”

It was also the kind of place you leave, he admits. He says his “brain” and his football talent, his ability to channel that “anger and aggression”, were his way out. Growing up, he knew he had a gift, an ability to see what was next, and he first joined Hertha Berlin at seven. In 2007 Tottenham Hotspur signed him for £5.4m: the “big move”, he says, but the wrong move. Left out, he wasn’t ready for London, and it could have ruined him. One morning, about a year later, he woke up, stood before the mirror, and the realisation hit him.

“I looked old,” Boateng says. He was 20.

“Every night I was out until six. I was like 95 kilos, swollen from the drinking and bad food. I said: ‘This can’t be me, I don’t want to be that guy. I have something inside: I’m a football player.’ I called my friends, two real friends, and they came. Together, we cleaned out my fridge and the house. That day, I said: ‘No, stop it.’ I didn’t drink. I didn’t go out. I started cooking; I wanted to eat healthily. From one day to the next.” Boateng clicks his fingers. “If I did it slowly, maybe I wouldn’t do it. I needed a clean break.”

How had it come to that? “Martin Jol told me he didn’t want me after a month. So, it became me against the world. You know when you shut off? That was me. ‘You don’t want me? I’ll enjoy life.’ I realise now how bad it was: six days a week nightclubbing, drinking for almost a year. But I was only 20. You don’t think things are going wrong. You see money coming in. ‘OK, I get my fun somewhere else.’ Girls, nightclubs, friends … Fake friends.”

The correction jars. There was an emptiness to life. “I left my home, family, all my friends, then my ex-wife left me and I was totally alone. I had friends but not real friends who’ll tell you: ‘What are you doing? Go and train.’ No. It’s: ‘Let’s go out.’” And at the time, [I thought] I needed that. The release, someone to talk to.

“Fans don’t care what’s in your private life, what happened in your past, where you come from. If you don’t perform they judge. I was the same, a fan judging Hertha Berlin players. That will never change. You’re a number in this system. You cost money, if you don’t work, they change the number. I had to learn to understand that; when I was 20 I didn’t.”

Boateng in action from Tottenham in 2007. Photograph: Keith Williams/Action Images

It is not just the fans. “In the team, everyone does their own thing; in the end, they don’t really care how you feel, why you’re sad or not training well,” he says.

Spurs’ former sporting director Damien Comolli once said Boateng was the one player he regretted signing, admitting he failed to spot a kid unprepared for the change or do enough to help. “It was probably our failure more than his,” he said. Boateng appreciates the sentiment now, saying it lifts a weight off; back then, he missed it. “No one came to ask: ‘How are you?’ No one. ‘How are you?’ Just one simple question: ‘How are you?’ No one, no one.”

Had they done, he might just have replied: “Fine,” not really aware of the damage being done, that things were even going wrong. “I was spending serious amounts: nightclubs, clothes, cars.” Three in one day, the story goes. “True,” he says. “Because you try to buy happiness. I couldn’t play football so I buy a Lamborghini. Wow, you’re happy for a week. After that you don’t even use it. Who drives around Loughton in a Lamborghini? I still have a picture: three cars, big house, I’m standing there like I’m 50 Cent. I look at it sometimes and say: ‘Look how stupid you were.’ But that made me who I am and I can look back and see it. I’ve learned. I grew up.”

“I woke up one morning, looked in the mirror and thought: ‘No, that’s not me, I don’t want to be that. I’m a footballer.’”

‘Klopp is the best coach in the world’

But footballers have to play and opportunities remained limited until Dortmund took Boateng on loan for six months in January 2009, the eve of their explosion. The manager was Jürgen Klopp and the mention of his name excites. “Yes!” Boateng says. “I could see it immediately. He’s the best coach in the whole world. He knows when to push you and when to comfort you. He knows when you need a drink, when you need water. He has this …” His voice trails off. “He has everything. Ask the players and they’ll say: ‘He’s the best, I’d die for him.’

“He knows exactly what every player needs and gives them time. There were players at Dortmund who played five minutes in six months but they were happy: happy to come to training, happy to work, because he made you feel important. Not necessarily as a player – maybe he doesn’t need you – but as a person. That’s why he’s successful everywhere. And Liverpool’s perfect; just watching his presentation you see it. ‘The normal one’: people there love that. If he’d gone to Paris, it would have been best suit, [different message]. He knows how to grab people.”

Moments, chance, decisions; they can change a career, a life. There must be times when Boateng thinks: “If I’d stayed at Dortmund with Klopp …” The response is immediate. “I’d have played a Champions League final, won the league, the cup. But: ‘if’, ‘when’ … I don’t know. I’ve had a career many dream of. I’m happy, but I know I could have done better; if I’d focused more, worked harder earlier. I’m happy to have met Klopp, to have worked with him, even if it was only six months.”

Dortmund wanted to keep him but not enough to match Spurs’ £4.5m asking price. Portsmouth came, with their invisible owner and impending crisis: a backwards step but a necessary one. “For almost three years, I hardly played; that was all I wanted. They said the stadium is small: ‘I don’t care.’ The pitch is bad: ‘I don’t care.’ They have no money: ‘I don’t care.’ The contract is this: ‘OK, I sign it. Just give me the ball, let me play.’ Portsmouth was small but real. It was crazy, beautiful. I loved playing there.”

Boateng surveys the surroundings at the Estadio Gran Canaria. Photograph: Carlos Díaz-Recio for The Guardian

His last season in England ended, perhaps inevitably, with relegation, and also an FA Cup final defeat against Chelsea, but he played and even got a semi-final goal at Wembley against Spurs – “a little payback to show them that I made mistakes but they made mistakes too,” he says. It is said without bitterness; that is just how it is. “You didn’t work, they send you away. But when you score against your old team after they didn’t treat you how you wanted, it’s an amazing feeling.”

“I played well at Portsmouth so knew I couldn’t stay,” he continues. What he did not know was where he was going but the 2010 World Cup in South Africa, where he faced his brother Jérôme against Germany, the country he represented at under-21 level, and reached the quarter-final with Ghana, whose passport he had acquired in May, meant he had a market. He was in demand again, a star. Even Mandela wanted him, he discovered.

“There were three people I always wanted to meet: Michael Jackson, Muhammad Ali and Nelson Mandela,” he says. “I only met one, and it’s hard to describe. It’s just joy. Mandela was in prison for 27 years just because he stood up for his rights and he sits there and has no anger inside. He should be angry with the whole world, but he wasn’t. He’s calm, just there in his little seat saying hello to everybody. He makes you feel calm. He was shining. It’s like a movie. It’s like an angel sitting there.”

And what did you say to him? Did you know what to say?

“Nooo! Luckily he broke the ice, because you just stand there. It was the World Cup, people were calling me ‘David Black-ham’, going crazy for me. I was kind of like a star. We go into the room: ‘Hello … hello … hello.’ He shook my hand, pulled me towards him and said: ‘My daughter wants to marry you.’ I said: ‘Sorry I already have a girlfriend.’ He said: ‘No, no but I have others, more beautiful.’ Everyone was laughing. The pity is we couldn’t take pictures because the flash hurt his eyes so I only have one.”

Prince stops for a second then laughs. “And it doesn’t even look like me…”

‘They were all there: Ibrahimovic, Seedorf, Pirlo, Ronaldinho …’

Soon after the World Cup, Boateng was on holiday when his agent phoned. He thought a deal had been done with Genoa – it had – but his agent asked if he fancied Milan. “I said: ‘Come on, are you kidding? Seriously? I’d love to,’” he recalls. “I went out partying. Next morning, he calls at eight. I was still tired. He said: ‘Get in the car, we have to meet.’ ‘Where?’ ‘Milan.’ He said: ‘You have trained, right?’ I said: ‘Of course,’ but it was a lie: I wasn’t training; I was enjoying my holiday. ‘Perfect, because I told them you’re an animal.’ ‘Yeah, yeah.’ Oh my God.”

“They were all there. I signed the day after Ibrahimovic; Robinho signed too. There was Seedorf, Pirlo, Ambrosini, Gattuso, Ronaldinho, Thiago Silva, Jankulovski, name it. My first day, I was there early doing the tests and saw the names. ‘This is a dream, this is a joke.’ I called my older brother: ‘I’m sitting next to Pirlo.’ ‘Take a picture, take a picture!’ ‘I’ve got David Beckham’s old locker.’ He’s like: ‘You’re lying’. I said: ‘I’ll send a picture.’”

Ronaldinho is always happy. You win, he’s happy. You lose, he’s happy. He scores three own goals, he’s happy

Zlatan was the most imposing. “You think he’s this arrogant, big fucker and completely not a nice guy but he’s the opposite: laughing all the time, cracking jokes. On the pitch, he’s very serious, very professional. But off it, the funniest guy ever.” So his persona is a facade? “Yeah of course because he doesn’t want to talk to you,” Boateng laughs. “So he puts that face on so you don’t even ask him a question.”

Do you see yourself in him?

“Not any more.”

Did you?

“Yeah, yeah. Because I was the same; I didn’t want to talk to people. I didn’t want to show them I had emotions. I [built] this big wall. But you grow up, you’re happier with life, you think: ‘Why not?’ Why not let people talk to you? Help them? Give them a smile?”

Speaking of smiles, Boateng rates Ronaldinho highest of all, his tone hushed. No facade here. “It’s genuine: he’s exactly how he looks. Laughing, smiling, all the time. Never, never, never serious. Impossible. He’s always happy. You win, he’s happy. You lose, he’s happy. He scores three own goals, he’s happy. He just wants the ball. Give him it. That’s why he was the best; he feels no pressure. And by then he had nothing to prove.”

Boateng at Milan: ‘This is a dream, this is a joke.’ Photograph: Marco Luzzani/Getty Images

It was no surprise to Boateng that Milan won the league; the surprise was that he was in the team that did it. “They had a superstar in every position but the crazy thing was I was playing. I started on the bench but fought my way in. Talent, technique – everybody there has that. Maybe the only one with less technique was Gattuso, but he ran 120 minutes like a psycho. I had to bring something different so I brought fighting spirit. Running, kicking, to the point where people said: ‘He’s the new Gattuso, the new Gladiator.’”

Title secured, all round to Silvio’s to celebrate. “No, no, no,” Boateng grins. “We knew it would come out, so we never went.” He continues: “[Berlusconi] was fantastic. We had a special relationship, a bond. He saw me as the little star he’d brought and encouraged me to draw out what I had inside. My best seasons were at Milan – and he was the one who pushed me. And he knows football; he won 30 titles. You read stories about him: he did this, he did that. OK. But when you meet him, he shines. He makes you feel you’re special, not him. That was his gift; that’s why they wanted him for president, why people voted for him.”

‘If you’re 18 you don’t know anything’

Six years from that Scudetto, his only major title, Boateng headed for Spain on a free. Las Palmas may seem a curious choice – this is only their second top flight season in 15 years – but, asked why, the answer is swift: “Why not?” Options were not limitless, he had seen them on television, and Mubarak Wakaso told him they were different – an attractive team whose idealistic manager Quique Setien emphasises expression and enjoyment. It’s also tempting to adapt the old joke: so, Prince, what was it that first attracted you to the holiday island of Gran Canaria?

It’s a grey morning at Barranco Seco, the training ground where flowers border a single pitch, and the cloud has not lifted when he arrives at the stadium in the afternoon, but he’s in shorts and wakes most days to sunshine. That matters. It seems he has mellowed – seeking sporting opportunity, sure, but life too. There’s a contentedness about him, comfort and acceptance, little impatience to move. No plans to return to England, either. He’s scored there and in Italy, Germany and Spain – the only current player to have done that – so he suggests: “Maybe I’ll go to France one day, add another.”

“England was good and you never know,” he continues. “But I feel really good here. It’s exactly what I need. I love the way football’s played. Real Madrid, Barcelona and Atlético dominate but we drew 2-2 at home against Madrid and won at Atlético. In Germany or England that’s impossible, not here. We have an incredible team, so much talent.” Given his reputation, Setien feared Boateng’s arrival but he’s been impressed and the Ghanaian fits in. That was best expressed at Villarreal when Pedro Tana completed a wonderful team move with a backheeled assist for Boateng to volley a goal worthy of Fifa’s Puskas award.

He thought so, certainly. In fact, he was counting on it. “I thought I was going to the Fifa gala. It was going to be the best day of my life after my kids’ birth. I always said I wanted to go, even if was just for the fair play award for owning up that it wasn’t a penalty. I felt if you go there, you’ve made it. You’re one of the best, even if it’s for a stupid thing, or for one goal one year. I thought they’d call me because I got so many messages and tweets …”

Boateng has become something of a mentor at Las Palmas. Photograph: Carlos Díaz-Recio/The Guardian

The call never came. “Someone explained that it’s not the best team goal, it’s the best individual goal. But football’s a team sport. Otherwise, I’ll play tennis. So, I saw all the messages and got excited.” Boateng is giggling now. “I even told my wife: ‘We’re going to Zurich.’ She asked me a week afterwards: ‘What happened to Zurich?’ I said: ‘Oh, forget about it.’ I’ll have to score another.”

He’s 29; there’s still time, but it’s also an age when retirement plans take shape. “My world’s football, my vision’s helping young players: where to put their money, which physio to see, getting them on the right path. Players need help and too many agents think about the quick money and they’re gone.”

“We didn’t study much, we’re not the best at maths or whatever, because we loved football. If you’re 18 you don’t know anything – and today at 18 you get five million net a year. You buy the world. That’s exactly what you think: ‘I. Can. Buy. The. World.’ I buy friends, I buy girls, I buy cars, I buy everything. I buy love, I buy happiness. That’s what you think. When you’re 18 you don’t care what your parents say, so you need this figure guiding you. I didn’t have that. So many players don’t.”

It’s role he’s already growing into. “There are young players here with so much talent. It can be difficult to live here: beautiful place, beautiful weather, you train two hours a day, you can go to the beach. To be focused on this island is difficult but I’m experienced, so I help, advise. If they listen, that’s up to them, but at least I can say I tried.

“I don’t want them to waste their talent. I’ve given them examples of things I did really wrong. I made mistakes in my life. I’m OK with that now, but I don’t want them to do the same stupid things that leave a mark forever: ‘Bad boy’, ‘drinker’, ‘party guy’. Some newspapers still have that image of me. Whatever. Come on, I spoke in front of the UN. Tell me another player who’s done that.”