This thread is a perfect illustration of the way shitty rumours about the Welsh language and Welsh attitudes are perpetuated.
The OP posts about something that seems - on the face of it - to be genuinely outrageous. That she isn't allowed to do business in Wales because she can't speak Welsh. 10 pages later, hidden in the depths of the thread, she admits that she was trying to set up a stall in an event organised by a charity that exists to promote and create spaces in which Welsh can be used as a social language.
Whether or not you agree with the aims of the charity in question, that is wildly different than a blanket ban on non-Welsh speaking stallholders.
But of course at this point, it's too late for anything like reasonable discussion and most people will never find that context. The thread is already full of stereotypes about aggressive, exclusionary Welsh speakers. It's already full of people complaining about how they're not against Welsh, not really, they just don't think it should be forced, they just think it should be something that the Welsh speakers manage for themselves.
Except when they do that, like, say by creating new organisations and that set up new events to celebrate their language, apparently THAT'S aggressive too.
About 15% of the population of Cardiff speaks Welsh. That's a minority, yes, but not a vanishingly small one. It's about 50k people. There IS an active Welsh-speaking community. So why do people like the OP think they don't exist? Well, partly because they don't notice the language being spoken in the background, but also because there aren't opportunities to do so where you can be sure that speaking Welsh is an option.
The OP knows that English is the default language in Cardiff, she mentions this often. Knowing to whom you can speak Welsh without being perceived as being "awkward" is a really nice and useful feature of events like Tafwyl.
It's also really useful for learners, who might not have the personal social links that fluent Welsh speakers do, and really struggle for opportunities to speak Welsh with other people outside of a classroom setting. For these people, being able to order a burger in Welsh - a simple, set interaction without the pressure of smalltalk, etc. - is actually a key REASON they might be attending such an event.
In terms of Employment Law, proficiency in Welsh would be a genuine occupational requirement for a customer-facing role in an event designed to provide a Welsh-language social space. There is no discrimination.
The Welsh language standards introduced by Welsh Government in respect of private businesses do not apply to businesses below a certain size specifically because of the disproportionate burden this would place on them.
You are not being discriminated against, you are being told that you are not able to provide the service that the event organisers require. And trust me, the number of burger-vendors or welly-boot sellers who ARE able to provide welsh-language service within the bounds of Cardiff isn't high. Their lives would be easier without this restriction.
In Cardiff a LOT of jobs have Welsh-speaking as a desirable criteria. No jobs, except the ones for which it is a genuine occupational requirement, list it as a requirement because it makes it really fucking difficult to hire people. I work in recruitment - I'm not speculating when I say this.