Hi Bhavya,
I don't quite agree with the Twitter card being important for SEO. It's a good meta tag to use, but doesn't affect your SEO scores as far as I know.
The language tag should be enough when provided on the HTML tag, however when using multiple languages very true you should define the current language.
Bhavya Shah
Born to help people
Chris Bongers There are also 2 important meta tags that are also highly important in SEO.
1) Twitter Cards meta tags & Open Graph Meta tags 2) Language Meta tags