fix(l10n): Fix plural issue with different locale and language

We need to use the language code here instead of the locale,
because Symfony does not distinguish between the two and would
otherwise e.g. with locale "Czech" and language "German" try to
pick a non-existing plural rule, because Czech has 4 plural forms
and German only 2.

Signed-off-by: Joas Schilling <coding@schilljs.com>

lib/private/L10N/L10N.php

@@ -215,7 +215,12 @@ class L10N implements IL10N {
 	public function getIdentityTranslator(): IdentityTranslator {
 		if (\is_null($this->identityTranslator)) {
 			$this->identityTranslator = new IdentityTranslator();
-			$this->identityTranslator->setLocale($this->getLocaleCode());
+			// We need to use the language code here instead of the locale,
+			// because Symfony does not distinguish between the two and would
+			// otherwise e.g. with locale "Czech" and language "German" try to
+			// pick a non-existing plural rule, because Czech has 4 plural forms
+			// and German only 2.
+			$this->identityTranslator->setLocale($this->getLanguageCode());
 		}
 
 		return $this->identityTranslator;

tests/lib/L10N/L10nTest.php

@@ -85,6 +85,15 @@ class L10nTest extends TestCase {
 		$this->assertEquals('5 oken', (string)$l->n('%n window', '%n windows', 5));
 	}
 
+	public function testGermanPluralWithCzechLocaleTranslations() {
+		$transFile = \OC::$SERVERROOT.'/tests/data/l10n/de.json';
+		$l = new L10N($this->getFactory(), 'test', 'de', 'cs_CZ', [$transFile]);
+
+		$this->assertEquals('1 Datei', (string) $l->n('%n file', '%n files', 1));
+		$this->assertEquals('2 Dateien', (string) $l->n('%n file', '%n files', 2));
+		$this->assertEquals('5 Dateien', (string) $l->n('%n file', '%n files', 5));
+	}
+
 	public function dataPlaceholders(): array {
 		return [
 			['Ordered placeholders one %s two %s', 'Placeholder one 1 two 2'],