Bella instinctively understands her new language abilities about as well as human beings instinctively understood their own language abilities three thousand years before Noam Chomsky was born! She knows nothing about etymology. She's not sure she knows any words, really, just... uses words correctly every time she tries to think of a sentence in the new language? She can produce rhyming sentences if she likes. She can think of words that sound like grash (horse fodder) such as glash (dagger) and maybe if she plays around with that enough she'll be able to produce a funny pun. She can talk like Fire Spell Guy did or talk like Stabby did, though she'd be hard-pressed to come up with any particular thoughts about the difference. If she tries to generate a third accent, she won't be ale to think of any.
The spellcasting language... she can remember some of the words Fire Spell Guy used. Ko-re-dagh, spirit of fire. Oir, mana. She does not know any other words in this language. She can't seem to imagine any new sentences in it. She can't rearrange words she remembers Fire Spell Guy using in a new pattern that feels grammatical.