htaccess language redirect when any language set - regex

Question

I have a website example.com, the default locale is English, I want to redirected to example.com/en If is not set a language.
All request without language argument should redirect to default language
sample :
example.com/blogpost/thisisapost
should redirect to :
example.com/en/blogpost/thisisapost
or
example.com/one/tow/three/etc
should redirect to :
example.com/en/one/tow/three/etc
The website Developed with Laravel.
<IfModule mod_rewrite.c>
<IfModule mod_negotiation.c>
Options -MultiViews -Indexes
</IfModule>
RewriteEngine On
# Handle Authorization Header
RewriteCond %{HTTP:Authorization} .
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
# Redirect Trailing Slashes If Not A Folder...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (.+)/$
RewriteRule ^ %1 [L,R=301]
# Send Requests To Front Controller...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^ index.php [L]
</IfModule>

Answer 1

You can create the redirect like this:
RewriteEngine On
rewriteRule ^/?(^((?!nl|en).)*$) https://example.com/en/$1 [R=301,L]
en and nl will not be matched now, but everything else will
You can see the result here

Answer 2

You can insert this rule just below RewriteEngine On line to make it work:
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule !^[a-z]{2}/ /en%{REQUEST_URI} [L,NE,R=301]
Explanation:
!^[a-z]{2}/ is negative condition that succeeds when URI doesn't start with 2 lowercase letters followed by /
/en%{REQUEST_URI} prepends REQUEST_URI with /en so that we get /en as default language in URLs
RewriteCond %{ENV:REDIRECT_STATUS} ^$ is a condition that checks for empty value of REDIRECT_STATUS variable to make sure this rule doesn't fire after an internal rewrite.