Reminds me of the morning in a small bed-and-breakfast where one Welsh-speaking guest was advised of the warm options (toast, white or brown, and/or eggs) and ordered Jyst tost, plîs -- brown - and I thought that an English speaker would have understood that order as well :)
I guess you could say "Dim ond un, os gwelwch yn dda" for sake of linguistic purity!
I am reminded of a story from a student at Bangor who I used to know. Apparently the Bangor student union had organised an event they called a "Mega-Rave Disco", and in the Welsh version of the publicity material they called it "Disgo Mega-Reif". As his account goes, a group of Welsh-speaking students complained to the student union about this, but their complaint got nowhere because the student union asked them to supply a better translation and they couldn't actually think of one.