The noun eisiau is not often pronounced as Duo's Gwyneth voice says it. You will normally hear people saying it as /isha/, /isho/, /eishe/ or something similar, depending on where they learned their Welsh. In informal usage some people will also write it as they pronounce it, such as isia, isio, etc.
in colloquial usage, the word-ending -au is often pronounced as /-a/ in north-west and south-east Wales and as /-e/ elswhere.
'A toast' is not really used in this sense in English (In Wales and the rest of Britain at least) - 'a piece/slice of toast' is the more more usual expression for a single item of toasted bread. The expected answers here are:
- I do not/I don't want toast
- I do not/I don't want any toast
'A toast' used in the sense of drinking a toast to someone (Cheers!, Your health!, etc) is llwncdestun. A common Welsh toast in this sense is Iechyd da! (Good health!).