The same problem occurs when trying to provide a translation for [[Wikimedia:Wikipedia-android-strings-year in review slide bytes added body/de]].

Sidenote: that error message is really unhelpful. Validators should explain why they are failing rather than all failing with the same unhelpful error message.

If you use the new TUX ui rather than editing the page through the wikitext editor, you get a better error message:

"Plural forms should be defined as {{PLURAL|one=…|…}}. This translation contains {{PLURAL|one=…|href=…}}."

Apparently some regex used in valdating translations gets confused by the raw HTML in the message.

It looks like this can be worked-around by making sure the raw HTML is outside the PLURAL template, i.e. wrap the PLURAL template only around the minimum required text to be pluralized:
https://translatewiki.net/wiki/Wikimedia:Wikipedia-android-strings-year_in_review_slide_most_read_english_articles_body/de

In T409655#11358674, @Dbrant wrote:

It looks like this can be worked-around by making sure the raw HTML is outside the PLURAL template, i.e. wrap the PLURAL template only around the minimum required text to be pluralized:
https://translatewiki.net/wiki/Wikimedia:Wikipedia-android-strings-year_in_review_slide_most_read_english_articles_body/de

Unfortunately, that will either not roundtrip, or even cause problems on export – the Android XML format doesn’t support the concept of “mostly the same, with one different sentence”; if it’s a plural message, it needs to be stored separately from the first character to the last, which corresponds to the whole message being in {{PLURAL}}.

In T409655#11358692, @Nikerabbit wrote:

https://gerrit.wikimedia.org/g/mediawiki/extensions/Translate/+/a8446a2457fa087bf227e9ff4b146404464b112b/src/Utilities/UnicodePlural.php#125 could be made more strict to avoid false positives with equals signs not close to a pipe. Just need to be careful to avoid letting true positives trough.

Actually, I think it’s a pretty easy change: adding an anchor at the beginning, i.e. changing ~\s*([a-z]+)\s*=(.+)~s to ~^\s*([a-z]+)\s*=(.+)~s. Everything that’s not at the beginning is not a plural form; and everything that’s HTML won’t be at the beginning (since the HTML tag will begin with a <, which isn’t matched by the regex, so the part matched by the regex won’t be at the beginning).

And looking at the code, I realized simply adding explicit |other= fixes the issue, maybe that could be mentioned in the error messages. (This may be something for T409665, though.)

In T409655#11359074, @Tacsipacsi wrote:

In T409655#11358674, @Dbrant wrote:

It looks like this can be worked-around by making sure the raw HTML is outside the PLURAL template, i.e. wrap the PLURAL template only around the minimum required text to be pluralized:
https://translatewiki.net/wiki/Wikimedia:Wikipedia-android-strings-year_in_review_slide_most_read_english_articles_body/de

Unfortunately, that will either not roundtrip, or even cause problems on export – the Android XML format doesn’t support the concept of “mostly the same, with one different sentence”; if it’s a plural message, it needs to be stored separately from the first character to the last, which corresponds to the whole message being in {{PLURAL}}.

It should round trip fine, but please let us know if it doesn't. Maybe the code that supports that is broken.

Change #1204752 had a related patch set uploaded (by Wangombe; author: Wangombe):

[mediawiki/extensions/Translate@master] UnicodePlural: Make plural form regex more strict

https://gerrit.wikimedia.org/r/1204752

In T409655#11359166, @Nikerabbit wrote:

It should round trip fine, but please let us know if it doesn't. Maybe the code that supports that is broken.

Ah, I wasn’t aware of rETRAe0afbd1eff5ff47688cb7b926a6f56d8227c5126. I don’t know of any actual breakage.

Change #1204752 merged by jenkins-bot:

[mediawiki/extensions/Translate@master] UnicodePlural: Make plural form regex stricter

https://gerrit.wikimedia.org/r/1204752

Change #1211039 had a related patch set uploaded (by Wangombe; author: Wangombe):

[mediawiki/extensions/Translate@master] Fix plural test case to handle zero item correctly

https://gerrit.wikimedia.org/r/1211039

Change #1211039 merged by jenkins-bot:

[mediawiki/extensions/Translate@master] Cover more edge cases in plural tests

https://gerrit.wikimedia.org/r/1211039