Are all the synonyms for this word accepted? Like, juvenile, puerile, others I can’t think of off the top of my head...
No, the course has no underlying automated thesaurus or dictionary. The range of Welsh and English vocabulary is based on that used in the already established, basic 'Welsh for Adults' courses taught in Wales.
Similarly, the course sticks to colloquial language forms and structures - Welsh has sometimes very different forms used in more formal speech and writing, and the course does not address those. Neither does it address any except very widely heard dialect structures and vocabulary.