I'm sorry, but I'm not really buying that there's any such distinction as 'high' and 'low' elven language, or even dialects (as we know them). If only for the fact that when Timmain (the most pure speaker of the elf language on the planet) came out of a 9000-10,000 year period of not speaking elvish at all, the first words she did speak were immediately recognizable and understandable by representatives of three elf tribes (who had no contact with each other for thousands of years), the trolls, and the Preservers. If there had been language shift during all that time, the assembled group would be going, "Huh? What?" Sure, the different tribes might have coined different words for different situations or events, or have slightly different accents; but that would be on the level of jargon or slang, not dialect.