But then again - what can you safely assume an average reader to know, and how do you determine that?
That's difficult if your story will be translated into several languages. What a European might know, might be totally unknown to a person in another continent.
One way might be to write foot notes - the other is to write a glossary, but both procedures have advantages/disadvantages. What about this: if you use a term you suspect someone might not know - why not include a small explanation via dialogue between 2 chars? And not the whole story, but the part you want to include into your story?
And another way ist this => give your story to another person and ask, if she/he knows what you are talking about. As for translations => maybe a short glossary might be appropriate then.
My favourite Australian fantasy youth book author is Margaret Mahy. She used Aborigine mythology and involved modern day kids into this world (a bit like Alan Garner). As my knowledge was very rudimentary in this case, I started to read these legends and found a totally new fascinating world of legends, gods and other creatures. So, her books opened a new world for me. But I don't know if I'm an average reader. I like to find out stuff - maybe a reader would just like to be entertained. In any case - she explained the part, where she put in a mythological creature very well. Her heroine simply memorised this.