"I do not want to work in a theatre."
Translation:Dw i ddim eisiau gweithio mewn theatr.
Sort of. eisiau and isio are different spellings of the same word (isio matches one particular pronunciation of the word spelled eisiau in the standard but, from what I have read, it is not pronounced exactly like that in any traditional dialect), and moyn is another word which is also used to mean "want", particularly in the south.
As for regional dialects - the issue is that many languages have a standardised variety that you can teach to foreigners, but Welsh does not; it's all regional dialects.
There have been attempts to standardise the language for teaching, e.g. Cymraeg Byw, which is where forms such as rydych come from.
Imagine that you have US English (with "color" and "gas") and UK English (with "colour" and "petrol")... but both in the same country so that you can't say that one is more or less right. Which form do you teach? Students will encounter both - so you basically have to teach both of them.
I do agree, but from a pedagogical point of view, when a new word is taught (even if it is a synonym), it is still better to introduce it and show how it works (yn or no yn ?)... rather than let it appear randomly in a strengthening lesson. I really hope it could be changed in the next tree, by putting them in specific skills at a more advanced stage of the learning...