Visual feedback when grabbing a selected piece of text in the target language column in the editor seems to suggest that it should work: The mouse pointer changes to a hand when clicking on a selected snippet of text and holding the mouse button. Dragging while still holding the button makes the pointer change to a hand with an arrow next to it and causes the cursor to follow underneath when hovering a text area, suggesting the text snippet is meant to be inserted at that point. Instead, upon releasing the mouse button, the text snippet remains in its original place, the selection gets dropped, and wikilinks disappear from it.
(I did not test yet what such text looks like afterwards in the wiki markup source code after "publish"ing - if e.g. there are some remnants of the wikilink left.)

Tested with Mozilla Firefox 38