Any info on very strange wing endings on this Stemme glider?
This was a test aimed at measuring the effect of feathered wingtips. The Stemme S-10 has a four-part wing, and for the experiment the outer wing panels were removed and replaced by five staggered small wings, as shown on the picture in your question. This reduces wingspan from 23&nbsp;m to only 11.4&nbsp;m plus the span of the wingtips, in total maybe 14&nbsp;m.

A feathered wingtip shows less induced drag than a straight wing of identical span, similar to a winglet, because it can involve a bigger mass of air into the creation of lift. But it cannot perform wonders, the effect is small.

The concept behind this experiment is called "winggrid" and the brainchild of Dr. U. La Roche. See here for one of the few still active sites on the topic - the original site http://www.winggrid.ch has long been abandoned.

Sad fact: This aircraft was destroyed in a mid-air collision in 2007. At that time, it had been reverted to the original wingtips.