They are not editable in the visual editor.
Topic on VisualEditor/Feedback
Confirmed. Example is [[:fr:|fr]] = fr - this should be an in-text link to the French Wikipedia.
Interwiki links are essentially templates (they abbreviate URLs), and so they are not currently editable (See VisualEditor#Status). Hopefully soon.
Actually, this isn't right; templates are very different, and will be editable very soon (indeed, you can use the experimental template editing code here on MW.org right now).
where is it
Go to a page with a template in - I've created VisualEditor:Template test as an example - note that editing the block template is currently broken due to a bug, which will be fixed tomorrow (sorry!).
Yes, sorry about this, I've created bug 49316 to make sure we get this done.