feat: reduce available languages per config value "reduce_to_languages"

Example: restrict to en, de, es, fr, it' languages

./occ config:system:set reduce_to_languages 0  --value  en
./occ config:system:set reduce_to_languages 1  --value  de
./occ config:system:set reduce_to_languages 2  --value  es
./occ config:system:set reduce_to_languages 3  --value  fr
./occ config:system:set reduce_to_languages 4  --value  it

Signed-off-by: Misha M.-Kupriyanov <kupriyanov@strato.de>

lib/private/L10N/Factory.php

@@ -628,6 +628,10 @@ class Factory implements IFactory {
 		}
 
 		$languageCodes = $this->findAvailableLanguages();
+		$reduceToLanguages = $this->config->getSystemValue('reduce_to_languages', []);
+		if (!empty($reduceToLanguages)) {
+			$languageCodes = array_intersect($languageCodes, $reduceToLanguages);
+		}
 
 		$commonLanguages = [];
 		$otherLanguages = [];

tests/lib/L10N/FactoryTest.php

@@ -672,6 +672,53 @@ class FactoryTest extends TestCase {
 		self::assertSame($expected, $result);
 	}
 
+	public static function dataTestReduceToLanguages(): array {
+		return [
+			['en', ['en', 'de', 'fr', 'it', 'es'], ['en', 'fr', 'de'], ['en', 'fr', 'de']],
+			['en', ['en', 'de', 'fr', 'it', 'es'], ['en', 'de'], ['en', 'de']],
+			['en', ['en', 'de', 'fr', 'it', 'es'], [], ['de', 'en', 'es', 'fr', 'it']],
+		];
+	}
+
+	/**
+	 * test
+	 * - if available languages set can be reduced by configuration
+	 * - if available languages set is not reduced to an empty set if
+	 *   the reduce config is an empty set
+	 *
+	 * @dataProvider dataTestReduceToLanguages
+	 *
+	 * @param string $lang
+	 * @param array $availableLanguages
+	 * @param array $reducedLanguageSet
+	 * @param array $expected
+	 */
+	public function testReduceLanguagesByConfiguration(string $lang, array $availableLanguages, array $reducedLanguageSet, array $expected): void {
+		$factory = $this->getFactory(['findAvailableLanguages', 'languageExists']);
+		$factory->expects(self::any())
+			->method('languageExists')->willReturn(true);
+		$factory->expects(self::any())
+			->method('findAvailableLanguages')
+			->willReturnCallback(function ($app) use ($availableLanguages) {
+				return $availableLanguages;
+			});
+
+		$this->config
+			->method('getSystemValue')
+			->willReturnMap([
+				['force_language', false, false],
+				['default_language', false, $lang],
+				['reduce_to_languages', [], $reducedLanguageSet]
+			]);
+
+		$result = $this->invokePrivate($factory, 'getLanguages');
+		$commonLanguagesCodes = array_map(function ($lang) {
+			return $lang['code'];
+		}, $result['commonLanguages']);
+
+		self::assertEqualsCanonicalizing($expected, $commonLanguagesCodes);
+	}
+
 	public function languageIteratorRequestProvider():array {
 		return [
 			[ true, $this->createMock(IUser::class)],