Double Language Website

You definitely do not need multisite since that would leave you with two sites having duplicate content and destroy your SEO even if they are in different languages.

Multilingual is what you are looking for, and there are various ways available for doing that:
https://www.google.com/search?q=multlingual+wordpress
https://ww.wp.xz.cn/plugins/search.php?q=multilingual

I have a site where I plan to use WPML, but I have no actual experience to share with any approach. But if you do not need to hire or use outside translators, that kind of connection or interconnection is one feature you will not need to consider.

Also, maybe take a look here if you need to be able to offer multilingual access at the Dashboard:
https://ww.wp.xz.cn/support/topic/two-wp-installs-with-same-database-content-and-users-1?replies=4#post-7014673
http://code.tutsplus.com/articles/new-wp-config-tweaks-you-probably-dont-know–wp-35396