"I like a party."
Translation:Dw i'n hoffi parti.
That is a local dialect form. There are so many dialect variations, especially of common verbs, that the course has to stick with only a limited number of them in order to be manageable.
Most variations are more in the spoken forms - when writing, most people stick to more standard forms.
I understand your point. Isn't there a difference, though, between (passively) allowing a form and actively teaching it? (The former would be fairly easily manageable, I would have thought.)
I think I'm far from alone in being a learner who mixes and matches widely (SSiW, duolingo, books, attendance at the likes of Sadyrnau Siarad if we're lucky enough to live in Wales, watching S4C, listening to Radio Cymru) -- but one thing that emerges pretty consistently is the advice (to those who live in Wales, at least): learn to understand and recognize the different registers and local variations but aim to be consistent in using just two (one spoken and one more formal/written). In the case of the spoken language, "fitting in" with the language used by those around you is particularly important -- though I might not go quite so far as one (university-based) tutor who regularly used to snap "Wi'n...!") at learners who said "Dw i'n..." !