If I am to interpret the wikipedia page, I think it's more common to talk about Hapax legomena within a single text -- or within a single author's work -- you could assert that a word was Hapax legomenon in the corpus of an whole language, but then you would have to have read the rest of the language's original recorded works, other people reusing the word from the original reference, and people overheard your own reference (assuming you broke the spell) without realizing they were breaking the spell.

How could you ever know if you were the one who ruined it, if it was or if it wasn't hapax when you first found it?