How to make WP in two languages (e.g. English & Bengali) without plugins

A way my agency has chosen to implement it in the past is through subdomains. We created the entirety of our English site as you would normally in WordPress on, used the Duplicator plugin to clone the site to, and then changed all the text on the new site.

And as @MarkKaplun recommends, a multisite setup would be appropriate here as well, though slightly more complex. It lets you keep the same WordPress core for both sites, but different content.

An option is to use a plugin like WPML (which is generally considered the best plugin to use if you ended up wanting to go that route). However, there are issues as pointed out by @MarkKaplun such as the integration with other plugins and WPML assuming all content should be the same.