yii\web\Request::getPreferredLanguage PHP 메소드

    public function getPreferredLanguage(array $languages = [])
    {
        if (empty($languages)) {
            return Yii::$app->language;
        }
        foreach ($this->getAcceptableLanguages() as $acceptableLanguage) {
            $acceptableLanguage = str_replace('_', '-', strtolower($acceptableLanguage));
            foreach ($languages as $language) {
                $normalizedLanguage = str_replace('_', '-', strtolower($language));
                if ($normalizedLanguage === $acceptableLanguage || strpos($acceptableLanguage, $normalizedLanguage . '-') === 0 || strpos($normalizedLanguage, $acceptableLanguage . '-') === 0) {
                    // en-us==en
                    return $language;
                }
            }
        }
        return reset($languages);
    }

 public function testPrefferedLanguage()
 {
     $this->mockApplication(['language' => 'en']);
     $request = new Request();
     $request->acceptableLanguages = [];
     $this->assertEquals('en', $request->getPreferredLanguage());
     $request = new Request();
     $request->acceptableLanguages = ['de'];
     $this->assertEquals('en', $request->getPreferredLanguage());
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de', 'ru-RU'];
     $this->assertEquals('en', $request->getPreferredLanguage(['en']));
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de', 'ru-RU'];
     $this->assertEquals('de', $request->getPreferredLanguage(['ru', 'de']));
     $this->assertEquals('de-DE', $request->getPreferredLanguage(['ru', 'de-DE']));
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de', 'ru-RU'];
     $this->assertEquals('de', $request->getPreferredLanguage(['de', 'ru']));
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de', 'ru-RU'];
     $this->assertEquals('ru-ru', $request->getPreferredLanguage(['ru-ru']));
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de'];
     $this->assertEquals('ru-ru', $request->getPreferredLanguage(['ru-ru', 'pl']));
     $this->assertEquals('ru-RU', $request->getPreferredLanguage(['ru-RU', 'pl']));
     $request = new Request();
     $request->acceptableLanguages = ['en-us', 'de'];
     $this->assertEquals('pl', $request->getPreferredLanguage(['pl', 'ru-ru']));
 }