Takashi Matsumoto and Haruomi Hosono faced a choice when starting a rock band in 1969: should the lyrics be sung in English, the genre’s lingua franca at the time, or Japanese? After a debate, the pair opted for their native tongue, and totally changed the course of their country’s music.

Their group Happy End – which also counted guitarist Shigeru Suzuki and guitarist/vocalist Eiichi Ohtaki as members – merged western-inspired folk-rock with Japanese vocals – a decision that has influenced everything from internet-embraced 80s “city pop” funk to modern J-pop. “My mother language is Japanese. If you translate it, that’s like adding filters,” explains the 74-year-old Matsumoto from a meeting room overlooking downtown Tokyo. “It will change the original meaning. Then it’s not my instinct or my words anymore.”

Between 1970 and 1973, Happy End released three albums, all recently reissued on vinyl. But the impact the quartet had has lasted for decades. Domestic critics celebrated the band’s works from the start, with 1971’s Kazemachi Roman hailed as a masterpiece. It still regularly tops lists of the best Japanese albums. “Musicians back then admired Cream, Jimi Hendrix, and so on. Everyone copied them and competed against each other,” says Hosono, who went on to even greater fame with the synthpop group Yellow Magic Orchestra. “I grew up in that scene, but with Happy End we decided to use our own words to create something original.”

Happy End emerged from the band Apryl Fool, which Matsumoto and Hosono played in. While their psych-rock sound helped them stand out at a time dominated by groups replicating the sounds of the Beatles and the Monkees, Apyrl Fool’s lyrics were still sung in English – all except for a two-part song written in Japanese by Matsumoto: The Lost Mother Land, about the rapidly changing state of his hometown Tokyo, and everything lost alongside it.

“The 1964 Tokyo Olympics changed so much,” Matsumoto says. “Rivers running around Tokyo were filled in. Highways and freeways were built. They used to have trams running along the area, but those had to be torn down for the roads. Things were changed by all that construction. It blocked out the sky: you can’t really see it anymore.”

Apyrl Fool wrapped up after singer Chu Kosaka landed a role in the Japanese version of the musical Hair. By that point, Matsumoto says he and Hosono were becoming more interested in west coast rock acts such as Buffalo Springfield, the Grateful Dead and Moby Grape, and wanted to create a new group inspired by that scene. Hosono recruited vocalist and guitarist Ohtaki, who brought a melodic edge indebted to the Beatles and the Association. “One day, Ohtaki called Hosono and said, ‘I finally understand what makes Buffalo Springfield good.’ That was the moment we all got on the same page,” Matsumoto says, laughing.

Guitarist Suzuki rounded out the lineup. “We expressed our thoughts honestly and passionately without being suppressed by adults. We were ‘pure’ – that is the word to describe who we were,” says Suzuki when asked how Happy End fit into the early 1970s Tokyo rock scene.

Their self-titled debut impressed critics with its combination of hard-edged sounds and Japanese sounds, and intensified the Nihongo Rokku Ronsō: the “Japanese rock controversy” revolving around whether it was possible for rock music to be authentic if it wasn’t sung in English. So Happy End returned to the studio for its second album looking to underline the point that Japanese rock could absolutely flourish. “We fine-tuned our strengths and figured out what directions we wanted to go … and dug into them,” Matsumoto says.

The resulting album, Kazemachi Roman, came to define the more confident Happy End. Matsumoto’s lyrics became more rich with detail, focusing on the pre-Olympics Tokyo he missed so much. But the strongest element is the sense of a band working together in close harmony. Matsumoto points to the album’s most famous song Kaze Wo Atsumete – which introduced the band to a wider international audience when it was featured in Sofia Coppola’s Lost in Translation – as exemplifying it: “It was like we were playing catch with one another – let’s change the lyrics, let’s change the melodies, we kept brushing things up.”

Happy End only found commercial success way later, but Kazemachi Roman impressed critics and the artistic community, and the “Japanese rock controversy” quietened down. “I think it’s great … like universally great,” Matsumoto says with a laugh. “We thought we had done the best we could, so after that we wanted to try different things, like more solo work. We didn’t spend as much time together. I regret that.” He playfully adds: “I feel like maybe Hosono just likes creating new bands. I was shocked once: we were on the Shinkansen [the bullet train] and I saw him writing new band names down.”

Happy End would record one more album – titled Happy End, like their debut – at Sunset Sound Studios in Los Angeles, but Matsumoto says it was clear everyone was focused elsewhere. “Everyone was thinking of the next step in their careers – that comes across clearly in the album,” he says. “It wasn’t a bad thing at all – it was natural.”

Suzuki would go on to have a rich solo career; Ohtaki became a beloved pop artist and producer, releasing work up until his death in 2013. Matsumoto became one of Japan’s most famous lyricists, working with big pop acts. Hosono did a little of everything. Later this year, a compilation with artists such as Mac DeMarco covering his work will mark the 50 years since his solo debut, and at 76 he continues to create, saying he’s hoping to start work on a new solo collection soon.

Happy End’s legacy, meanwhile, feels even more prominent in the 21st century. “Before, all the new music had to go through the US first, and then you would catch it after,” Matsumoto says. “But now, because of the internet or Spotify, you can find all this fun and interesting music from any country.” Modern music is global, and no longer dominated by any one place or dialect. Happy End argued for the power of the Japanese language, but more generally the importance of simply expressing yourself however fit best. “I believe you should speak with the language you feel the most comfortable expressing yourself with. That goes for any country in the world, not just Japan,” Matsumoto says. “There are no filters.”