Redirect root domain to subfolder - .htaccess

Question

Here is our current .htaccess file with the rules we need to keep but also we need to add a new rule that redirects from the root domain to a subfolder URL
example.com -> example.com/fl/en.html..
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.co\.nz$
RewriteCond %{REQUEST_URI} !^/subfolder [NC]
RewriteRule ^(.*)$ /subfolder/$1 [L]
RewriteEngine on
RewriteCond %{REQUEST_URI} ^(/home.html|/info.html|/flash|/external)
RewriteRule (.*) http://example.com/fl/en.html [R=301,L]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Answer 1

There doesn't appear to be anything particularly special required, providing you put the directive in the correct place. To redirect the document root (for example.com) to /fl/en.html in .htaccess can be done like so:
RewriteRule ^$ /fl/en.html [R,L]
This just needs to go after the directive that rewrites everything for the host domain.co.nz and before your front-controller. (You could potentially combine this with your existing directive that redirects /home.html, /info.html, etc.)
Your existing rules could be further optimised. Instead of checking the requested URL-path in a RewriteCond directive and allowing everything through in the RewriteRule pattern, it is more efficient to do what you can in the RewriteRule pattern first (since this is what is processed first).
Also, since you are using WordPress, any custom directives you add to .htaccess should be outside of the # BEGIN WordPress section. WordPress itself maintains this section, so any manual customisations you make could be overwritten during an update.
Also, there is no need to repeat the RewriteEngine directive. (The last instance of this directive wins and controls the entire file.)
So, bringing all this together, we have something like:
# Rewrite all requests for domain.co.nz to subfolder
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.co\.nz$
RewriteRule !^subfolder /subfolder%{REQUEST_URI} [L]
# Redirect document root to /fl/en.html
RewriteRule ^$ /fl/en.html [R,L]
# Redirect specific paths to /fl/en.html
RewriteRule ^(/home.html|/info.html|/flash|/external) /fl/en.html [R=301,L]
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
Note that the above directive (with a single R) is a temporary (302) redirect. Change this to R=301 if this is intended to be permanent, but only once you have tested that it's working OK (to avoid caching issues).
As always, make sure you've cleared your browser cache before testing.