This is likely due to the fact that elementary is running the Pantheon desktop not Gnome Shell. While Pantheon is built upon GTK3, it is not a Gnome Shell desktop. Therefore many things that work in Gnome Shell will not work in Pantheon.
I'll try to help with this one:
First, obviously, enable the extension in Code's Preferences.
Now, right-clicking inside a document will let you choose from one of the pre-existing languages:
If, like me, you might need a language that is not in the list, you can easely add it if you have the available dictonary files. The extension 'looks' for available ...
Nautilus and Files do not share any code. Any extensions written for Nautilus are not compatible with Files.
There is a package called "webcontracts" that provides Pastebin through a system service called Contractor among other things. You can download a .deb from the link or install through the unstable daily PPA.