Plaid Cymru and its leader, Rhun ap Iorwerth, made political history this month: they won the Senedd. For the first time ever, Wales now has a progressive majority that is not dependent on Labour. Polls had put Plaid and Reform UK neck and neck. In the run-up to the election, some of my Welsh friends were panicking. They were relieved that Reform came second.

I was never convinced that Reform’s brand of essentially English ethno-nationalism was ever going to triumph in Wales. The party seemed to think it could transpose its tactics from next door and that they’d work in the same way. Yet unlike Plaid, Reform UK has no story to tell about what it means to be Welsh.

Plaid’s victory brought to mind an incident that took place around the time of the Brexit referendum. Days before the vote, a Muslim woman on a rail replacement bus from Cardiff to Newport was allegedly told by a racist to “speak English,” only for another passenger to point out that she was speaking Welsh. That such overlapping identities could exist was beyond credulity to some at the time. Yet in the May elections, voters made a clear statement against the sort of politics that demands that a stranger on a bus “speak English”, just as they did in the Caerphilly byelection last year.

Plaid Cymru – despite being a “nationalist” party – represents a diverse and inclusive Wales that is forging ahead with its own idea of national identity against a rising tide of rightwing populism. You don’t have to be a Welsh speaker to be part of the project. Plaid won even in areas with historically low numbers of Welsh speakers, such as Ebbw Vale.

It wasn’t always like this. Plaid used to represent more of what you’d call a classic nationalism: one predicated on quite a limited idea of Welshness and rooted in English colonial attempts to eradicate the language and culture. That history is important to so many of us, particularly the impact of second homes on small rural Welsh communities which continues to exacerbate the housing crisis, but it didn’t always speak to the whole country. Growing up in Gwynedd, where Welsh is spoken widely, I was sometimes made to feel as though I wasn’t “Welsh enough” because I wasn’t born there. This is despite the fact that I have a name straight from the Mabinogion (Welsh prose tales from the 12th century), speak the language fluently, sing Welsh lullabies to my son, and have ancestors who died in the quarries that loomed over my childhood home.

There was a feeling among some people living in Wales that Plaid was a party that catered to white native Welsh speakers. The past decade has seen the party work to shake off this reputation and embrace a broader civic nationalism, one which includes a fight for social justice and self-determination for anyone who calls Wales home.

When I interviewed then-leader Leanne Wood in 2015, she said that “I’ve been really clear about our project being a civic-based project for everybody who lives in Wales. All people have a stake here, if they live here, and nationality, identity – those kinds of questions – they’re not really important from a political perspective.”

I think what Wood – a socialist who learned Welsh as an adult – recognised was that Wales needed to tell a new story about its national identity, one in which being Welsh isn’t about whether you can pronounce every word perfectly, know the grammar inside and out, or have two white Welsh parents.

You don’t have to go far to find an alternative vision of Welshness. The journalist Seren Jones has a brilliant video in which she points out that Cardiff has one of the oldest Black British populations in the UK and corrects some of the misconceptions she often encounters about Wales. Letting off a magnificent stream of “Wenglish”, the sort that might have once made a Plaid old boy cringe, she makes the case for this alternative vision of Welsh identity.

Most Welsh citizens will have had some exposure to the language – it is part and parcel of living there, even if you don’t speak it day-to-day. There’s a drive towards encouraging more learners and I expect that’s playing a role in the fight against Reform. Having more than one language has been associated with more welcoming attitudes towards immigrants.

Reform’s story, in contrast, involves more of a classic nationalism: triumphalist, xenophobic, always hypervigilant to some foreign threat. It’s identity through exclusion. As Yuliia Bond, a Ukrainian refugee who lives in Caerphilly reflected last year: “Reform UK tried to create panic and hate with tactics used not only in the UK, but by far-right political parties all over Europe and across the world. The messages they used in Caerphilly didn’t feel local. They felt imported – like someone copied a script from another country and dropped it through our doors.”

For Reform, politics isn’t a case of who you are, but of who you aren’t. To quote Umberto Eco’s essay, Ur-fascism, “the only ones who can provide an identity to the nation are its enemies”. As he goes on to say, “the followers must feel besieged” – a better summary than this of Reform’s political ethos is hard to find.

The confidence with which Plaid has tackled Reform has been inspirational. Labour has always been queasy about Welshness and complacent about its dominance, so failed to notice that a new Wales was taking shape around it. Meanwhile, Reform is trying to inject a distinct “Welshness” into its political brand (whether this proves to be effective remains to be seen). Now it’s up to Plaid tocontinue to stave off the threat through inclusivity, acceptance – and a rejection of narratives of “invasion”.

• Rhiannon Lucy Cosslett is a Guardian columnist