.htaccess subdomain languages
If your default url starts with www (like www.domain.com) then your translated urls will look like en.www.domain.com.
But if you try to login en.domain.com, you may think that your plugin doesn't working.
Before try anything be sure that any subdomains shows your WordPress homepage. With default settings WordPress should not redirect to you anywhere.
Then set your default url without www prefix (for ex. domain.com)
And check the permalink finally
If you fail at any steps, disable all plugins and even change your theme to default one.
You'd have to add wildcard domain setting at your DNS settings for the domain. https://codex.wordpress.org/Configuring_Wildcard_Subdomains
Wildcard domain redirect with WordPress See also this post.