Adding a link with an interwiki prefix doesn't do what I expect it to do.
In the English Wikipedia I type :fr:ASCII in the link inspector, but what I actually get when I save is fr:ASCII. The software thinks that it's an interlanguage link rather than an inline interwiki link to an article in the French Wikipedia, and doesn't show it in the text.
The colon in the beginning must not be removed if it was typed by the editor.
If you paste the full external link Parsoid will abbreviate it to an interwiki link. What could be improved is an interface for adding such links with autocomplete.
Copied my comment from T188871 which is a duplicate of this task I think:
Yes, it would be nice to have autocomplete too for people who do not yet know the page name.
Suggested look and feel:
(I "stole" the dropdown appearance from https://stats.wikimedia.org/v2/#/all-projects .)
The original bug here was that writing :fr:ASCII into the link inspector gives you fr:ASCII instead of :fr:ASCII. I just tested this, and this bug appears to be fixed, so I'm closing as resolved.
@Gryllida What you're requesting seems to be a separate thing from this, since this was a bug report and you're making a feature request. It should probably be in a different task.