If I can chip in....Most polyglots (of which I am not one) will agree that the hardest part of learning a language is the vocabulary. At least thats what Steve Kaufmann and Richard Simcott, both renowned polyglots say. I would say, without a shadow of a doubt this has been true for me. It's been a hard slog but I realised that I just had to sit down with a good novel and my dictionary app and slug it out. That is how I acquired a lot of my vocab. Its been hard, especailly when most of my exposure to welsh has been through radio and S4C, so I don't get the audio reinforcement that people surrounded by Welsh speakers do. Also as there are fewer shared cognates with the romance languages or Germanic languages, the vocab is much, much less intuitive compared to Spanish, which I am learning at the moment.
Considering this, I have to say that it is rather frustrating to come across speakers who say things such as 'suggesto' or 'rehearso'. And when did 'iwso' become so popular!? I heard it on Un Bore Mercher and thought, what the hell, it's now in the main stream Welsh media!? Welsh has beautiful words and sounds, which learners works hard at to learn. I think they should be used orally and not confined to written Cymraeg.
Oh well as the saying goes.
Gwell Cymraeg slac na Saesneg slic. (irony?)