This is a Parsoid bug:

$ echo '<a rel="mw:WikiLink" href="./de:Foo">de:Foo</a>' | node tests/parse.js --html2wt
[[de:Foo]]
$ echo '[[de:Foo]]' | node tests/parse.js
<link rel="mw:PageProp/Language" href="//de.wikipedia.org/wiki/Foo" data-parsoid='{"stx":"simple","a":{"href":"//de.wikipedia.org/wiki/Foo"},"sa":{"href":"de:Foo"},"dsr":[0,10,null,null]}'/>

Related to T136653: Parsoid doesn't recognize interwiki shortcuts in the href attribute ?

I've tried to read some code to see if there's a simple fix for this. It seems that Parsoid attempts to handle this in at least two places in LinkHandler.js:

			// Ensure that we generate an interwiki link and not a language link!
			if (rtData.isInterwikiLang && !(/^:/.test(target.value))) {
				target.value = ':' + target.value;
			}

			if (linkData.isInterwikiLang && !/^[:]/.test(linkTarget) &&
				linkData.type !== 'mw:PageProp/Language') {
				// ensure interwiki links can't be confused with
				// interlanguage links.
				linkTarget = ':' + linkTarget;
			}

Clearly, neither of these does the job. But I'm not familiar with the code and can't easily see why these checks are not reached (?) and I don't want to spend too much time diving into this right now.

I actually kept digging a bit more. I blamed these lines to find where they came from.

The second has been there since forever (rGPARb8eb09f6adce: Handle local interwiki links.).

The first one is more interesting, added in 2017 (rGPAR95155bab9ea9: T136653: Handle interwiki shortcuts). That commit looks like it was supposed to resolve this exact bug, and also comes with unit tests that prove it should work. So why doesn't it?

The supported HTML syntax, copied from the unit test, is like this: <a rel='mw:WikiLink' href='./fr:Foo'>Foo</a>. VisualEditor generates slightly different HTML: <a href="./Fr%3AFoo" rel="mw:WikiLink">Foo</a>. The href attribute is percent-encoded and capitalized. I tested on https://en.wikipedia.org/api/rest_v1/#/Transforms/post_transform_html_to_wikitext and it turns out that it's the percent-encoding causing it to not work!

@ssastry @cscott Should Parsoid also support percent-encoded hrefs here?

I worked around this in VE: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/VisualEditor/+/525682