If you worked in an office where everyone except you speaks Welsh and all meetings have to be in English just for your benefit, wouldn't it be better to learn Welsh rather than change the office language? Refusing to would be rude in my opinion.
Obviously not an option for the OP who's only visiting.
Who said I was refusing? Perhaps in this situation I'm visiting from another office, or have just transferred and my Welsh isn't great yet? Perhaps I have a grasp of the language but not enough to follow the intricacies of a meeting? There are many reasons why one person in a group may not speak the native language but if they have a need to follow the conversation, not just a want, and the only way that will happen is for everyone to speak another language it is common courtesy, never mind common sense, to speak in the language everyone understands.
When I worked in Spain, I first went over with a decent level of Spanish and could more than hold my own in a general conversation. When I first started working in the English Department of my school, they conducted the meeting in Spanish, when 90% of the participants were native Spaniards and the others were fluent ... halfway through the first meeting I was totally lost - the rapidfire Spanish, lots of different people talking, and the technical terms were just too much for me to cope with translating in an official setting where I needed all the information being discussed. Fortunately for me my colleagues all realised I couldn't yet keep up and offered to switch to English as they could all speak it to a fluent level.
They could have continued to converse in Spanish, as it was my responsibility to learn to speak Spanish in Spain after all, but they acknowledged that to do so purely on principle - when they could all speak in English fluently - would be counter productive as I wouldn't get the information I needed to do my job properly nor could I pass on my information clearly enough for them to action it, and the come back for that would be on the department as a whole, not just on me. Six months later and our meetings were more or less always held in Spanish because, by this point, I had enough of a technical grasp of the language to be able to understand everything ... but whenever any new teachers/guests/invited speakers/TEFL reps came to meetings we always asked them which language they would feel most comfortable in.
What exactly is RUDE about receiving an invite in another language though?
Thoughtless, perhaps, but how is it rude and/or offensive?
It's rude for a host to send an invite that they know the invitee won't be able to read (Google translate notwithstanding) as it comes across as a slight of "we're not bothered enough about you attending to even provide the information in a way you can understand. If you want to come to our event then you have to do all the work to facilitate your attendance." Especially for an invitation to a wedding where there is an expectation of the guest bringing a gift, it's rude to send an invite that suggests you're not fussed whether or not they attend as it looks a bit gift-grabby.
If you want someone to come to your event then it is your responsibility to ensure that the information is provided in a way they can understand. It would be one thing if a host sent an invite in Welsh and didn't know that their guest didn't speak the language, but if they know that the invite won't be understandable then they should also provide the information in another way too. Even if that's just a post it note attached to the official invite with: Wedding date: 12th of June; Service: 10am Holy Cross Church, Aberystwyth; Reception: 1pm Park Lodge Hotel