TYPO3 Core

Indeed. Actually the Xliff parser is respecting the pluralize in this format:

<group restype="x-gettext-plurals">
    <trans-unit id="label.pollsFound[0]">
        <source>Found %d polls.</source>
    </trans-unit>
    <trans-unit id="label.pollsFound[1]">
        <source>Found one poll.</source>
    </trans-unit>
</group>

But the LocalizationUtility in Extbase does not take the numeric arguments into account, when identifying the label.
Instead the [0] is hardcoded.

It's about these lines of code in \TYPO3\CMS\Extbase\Utility\LocalizationUtility::translate:

    // The "from" charset of csConv() is only set for strings from TypoScript via _LOCAL_LANG
        if (!empty(self::$LOCAL_LANG[$languageFilePath][$languageKey][$key][0]['target'])
            || isset(self::$LOCAL_LANG_UNSET[$languageFilePath][$languageKey][$key])
        ) {
            // Local language translation for key exists
            $value = self::$LOCAL_LANG[$languageFilePath][$languageKey][$key][0]['target'];
        } elseif

Before this if-condition, we could check the first item in $arguments. If it's numeric apply Xliff pluralization.
I'm not sure how complex this pluralization is to implement. But even if we would just differ between "1" and "n" entries, it would fit for most languages.

According to this document http://cldr.unicode.org/index/cldr-spec/plural-rules we should at least implement:

• zero/default [0]
• one [1]
• other [2]

• "other" is used, if given
• otherwise zero/default

• "one" is used, if given
• otherwise zero/default

• "zero/default" is used

The key 0 is always given, also when a translation has no plural forms.

Here, I've copied the LocalizationUtility and added a few changes:
https://bitbucket.org/t--3/t3oodle/commits/e2d628861684804067570fcab715350a97c0db4e#chg-Classes/Utility/LocalizationUtility.php

The changes are wrapped with // TODO: START MODIFICATION and // TODO: END MODIFICATION to make it easier to spot.

• supports the plural forms "zero", "one" and "many".
• makes it possible to get overwritten by typoscript, with additional keyword "_pluralize":
  

   plugin.tx_myext._LOCAL_LANG.de.label.itemsFound._pluralize {
      0 = Nothing to see
      1 = Just one item found
      2 = %i items found
    }
  

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 11 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 12 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911

Patch set 13 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/64911