To give additional info for others who may find this thread — currently to drag and drop in 5.5 you cannot have Top Toolbar turned on, as the method described by @clorith here only works via the inline block-level toolbar.
Thanks for this info! For reference, if useful for others like me who are starting to poke around with the plugin, here is the full list of available language classes in Prism.js: https://prismjs.com/#supported-languages