Read to the end. It's worth the journey.
I decide if sweet or stinky
There is a letter which makes the difference between "SWEET" and something stinky ("SWEAT"). It is A (or E).
For most locks I'm the key
"E" and "A" seen as keys on a keyboard.
Go ahead and keep on piling
because even a child can find me
Keep on piling means collecting/counting which every child can do. "E" happens to be the most used letter in this riddle.
It's no vanishing
Keep this is mind... we need to get rid of "ITSNO" from somewhere.
no clues here
no hiding there
To be taken literally.
It's no use
Oh, let's get rid of it now. "ITSNOUSE" -> "USE".
A man of words
An educated man, an author or poet, of the Enlightenment era (see title) who is German (see preface). See here for a list.
participating right now
Participating presently... There aren't that many German Enlightenment authors, "LESSING" is one of them and his name reminds me of a present participle (it is).
Teach him a lesson
and break his literal vow
This is too tricky, we remove the "ING". Because it's what makes "LESSING" a verbal part - it is his vow, his constraint. So we have "LESS" (there is also a small hint in Teach him a lesson giving you lesson >> lessen = take something away).
Subdued by ships
Ships subdue the "SEA"/"C" and "PEOPLE" perhaps.
Along lonely isles
Simply the letter C (="sea") along lonely isles " : ".
I may result
in stunning smiles
C: The word may also allows for something that is not smiling: :C
That is to say:
We have "C"
So we end up with:
E USELESS C
Now taking away what's useless,
Which sounds like:
Now, look in the preface again:
Well, as you might have thought...it's easy.
Lynch mob time!!