Determining which statistics and creating a technical definition of "desired tech stack" might be where I would start. Though I'd be the first to admit that where I would start may be orthogonal to sensible.
Jobs ads for example: "whatever position" but keywords : ruby, rails, ember, hula, dancing etc @ Berlin
What people search for
anything else ?
Thanks for the answer. Unfortunately, I still don't understand what exactly the question is about.