Translation:Nos da, hwyl
I am rather surprised to see "Goodnight" spelled as one word, yes in British English it is regarded as one word but in US English, which Dewi Lingo teaches, it is regarded as two words.
The Welsh course uses British English because its intended mainly for people in Britain (i.e. where Wales is), because they are more likely to want to learn Welsh and actually get opportunities to use Welsh than people in America (unless they live in Welsh Patagonia but that's in Argentina not the US and they speak Spanish so wouldn't be using this course).
I mean, it's not like the course rejects US English, it just favours British English.