When inserting a link to an existing wiki page or external link, the display text (= label) in the resulting link is forced as the plain technical string of the page or file, including the namespace etc. For external links on mediawiki.org it is shown as "". Changing the link display text (= label) is only possible in a separate steps afterwards. For pages, after saving the link (closing the linking dialog box), it is possible to go into the middle of the current display text (= label), type the correct display text and delete the originally inserted text before and after the corrected text (which is awkward). For external links, displayed as "" I have not managed to change the display text in this way. However, when (and only when) opening the link a SECOND time for editing, a button "add label" appears, allowing to do this. After now showing the full URI, it is again, as above, possible to correct the display text.
SUGGESTION: In linking dialog, always support a second text field for the display text/linking information, both when creating a link and when editing it. Compare how easily and intuitively this is handled in Google docs.