I agree with those who say this is likely a clear indication that the ceremony will be in Welsh. In their place I would likely have scribbled a quick translation on notepaper, but I don't think it's terribly rude either.
I do think the fact this thread has run 17 pages says something about how oddly threatening some English people find Welsh. There's this idea that because you can speak English, speaking any other language in the presence of an English speaker is somehow rude or deliberately exclusionary. I think in many instances this stems from not having lived in a bilingual culture.
When you have a relationship with someone and you are used to speaking one language with them, it is really, really, really weird to switch to another. It feels very odd and "fake". In some cases - with elders - it can even feel disrespectful because of the cultural issues at play. So even in situations where it clearly isn't disrespectful, and where everyone is fine with - in theory - switching to English - it's WEIRD and it's very easy to switch back without thinking because that's what's natural.
I have a friend with whom I always speak Welsh. Her partner doesn't speak Welsh. When the three of us are together, I address him in English, and I speak English if it's a conversation we're all participating in. If we're sat at the table having a coffee and chatting and he's surfing the internet on the other side of the room, then we speak in Welsh. We're not trying to exclude him, but there's no need for both of us to feel awkward expressing ourselves just because he happens to be in the general vicinity.
And that's where the confusion comes in, I think. A lot of English people seem to assume the only reason for this behaviour is to secretly talk about others present. It's not.
The idea that everyone should speak English for the comfort of a single non-Welsh speaker is based on the assumption that it's equally easy for the Welsh speakers to switch to English. It isn't. So whose comfort do we prioritise?
The other problem is the fact that Welsh is a minority language. If it should never be spoken if anyone present can't understand it, then the places it can be spoken will be severely curtailed. It's effectively an assault on its survival. I know that's not the intention, but that's the effect.