.htaccess can't add directory slash - php

Question

So I've been trying to rewrite my php URLs with .htaccess so that they are more SEO/user friendly. Here is my code:
RewriteBase /
Options +FollowSymLinks -MultiViews
DirectorySlash On
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^([^/]*)/$ /?lang=$1 [L]
RewriteRule ^([^/]*)/contact$ /contact?lang=$1 [L]
What this does is, it turns this:
http://www.example.com/?lang=en
http://www.example.com/contact?lang=en
Into this:
http://www.example.com/en/
http://www.example.com/en/contact
The problem:
http://www.example.com/en/
is not the same thing as:
http://www.example.com/en
And I get a 404 Not Found error when I try to point my browser's address to http://www.example.com/en. How can I make both versions of this work?
Adding DirectorySlashes On to my root .htaccess did not work. But my mobile version located in the /m/ folder has the following .htaccess:
# Set dir index
DirectoryIndex index.php
# Set dir slashes
DirectorySlash On
And it works! When I visit http://www.example.com/m I get redirected to http://www.example.com/m/. That is the wanted behavior. I guess it works because /m/ is a sub directory...
The question:
How can I make http://www.example.com/en redirect to http://www.example.com/en/ without breaking the rest of my URLs?
Edit (my entire .htaccess):
# Do not remove this line or mod_rewrite rules and search engine friendly URLs will stop working
RewriteBase /
# Disable directory views
Options All -Indexes
IndexIgnore *
# Follow Symbolic links
Options +FollowSymLinks -MultiViews
# Create Directory Slashes
DirectorySlash On
# Rewrite urls
<IfModule mod_rewrite.c>
RewriteEngine on
# Pretty desktop URLs
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^([^/]*)/$ /?lang=$1 [L]
RewriteRule ^([^/]*)/prices$ /prices?lang=$1 [L]
RewriteRule ^([^/]*)/offers$ /offers?lang=$1 [L]
RewriteRule ^([^/]*)/maps$ /maps?lang=$1 [L]
RewriteRule ^([^/]*)/contact$ /contact?lang=$1 [L]
RewriteRule ^([^/]*)/links$ /links?lang=$1 [L]
RewriteRule ^([^/]*)/error$ /error?lang=$1 [L]
RewriteRule ^([^/]*)/verify-new$ /verify-new?lang=$1 [L]
RewriteRule ^([^/]*)/verify-old$ /verify-old?lang=$1 [L]
RewriteRule ^en/ski-rent-prices$ /ski-rent-prices [L]
# Rewrite mobile URLs
RewriteRule ^([^/]*)/m/$ /m/?lang=$1 [L]
# WWW
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
# Rewrite URL without file extensions
# For .PHP files
RewriteRule ^error$ error.php [L]
RewriteRule ^offers$ offers.php [L]
RewriteRule ^prices$ prices.php [L]
RewriteRule ^maps$ maps.php [L]
RewriteRule ^contact$ contact.php [L]
RewriteRule ^links$ links.php [L]
RewriteRule ^verify-new$ verify-new.php [L]
RewriteRule ^verify-old$ verify-old.php [L]
# For .PDF files
RewriteRule ^ski-rent-prices$ ski-rent-prices.pdf [L]
</IfModule>
# Add Custom Error pages
ErrorDocument 400 /error.php
ErrorDocument 403 /error.php
ErrorDocument 404 /error.php
ErrorDocument 500 /error.php
# Default Character set
IndexOptions Charset=UTF-8
AddDefaultCharset UTF-8
# Hide the server signature
ServerSignature Off
<IfModule mod_rewrite.c>
# Add correct content types for documents
# JS:
AddType text/javascript .js
# CSS:
AddType text/css .js
# TXT:
AddType text/plain .txt
</IfModule>
<IfModule mod_headers.c>
# IE Compability Mode
BrowserMatch (MSIE|Trident) ie
Header set X-UA-Compatible "IE=Edge,chrome=1" env=ie
# Disallow 3rd party iframes
Header always append X-Frame-Options SAMEORIGIN
</IfModule>

Answer 1

Rewrite --- www.example.com/?lang=en => www.example.com/en/
RewriteCond %{QUERY_STRING} (^|&)lang=en($|&)
RewriteRule ^$ /en/? [L,R]

Answer 2

Updated code for testing
RewriteBase /
Options +FollowSymLinks -MultiViews
DirectorySlash On
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ /$1/ [L,R=301]
RewriteCond %{REQUEST_URI} !^(.+)/(.+)/$
RewriteRule ^(.+)/ ?lang=$1 [L,R=301]
RewriteCond %{REQUEST_URI} !^(.+)/(.+)/m/$
RewriteRule ^(.+)/(.+)/ $2?lang=$1 [L,R=301]
RewriteRule ^(.+)/(.+)/m/ $2/m/?lang=$1 [L,R=301]
See the browser address bar and check if the redirections are correct. If it wrong provide the source link and redirected link. If it is ok, Then use the below code.
RewriteBase /
Options +FollowSymLinks -MultiViews
DirectorySlash On
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ /$1/ [L,R=301]
RewriteCond %{REQUEST_URI} !^(.+)/(.+)/$
RewriteRule ^(.+)/ ?lang=$1 [L]
RewriteCond %{REQUEST_URI} !^(.+)/(.+)/m/$
RewriteRule ^(.+)/(.+)/ $2?lang=$1 [L]
RewriteRule ^(.+)/(.+)/m/ $2/m/?lang=$1 [L]
I have tried the code with the below links and its working fine.
http://www.example.com/en
http://www.example.com/en/
http://www.example.com/en/contact
http://www.example.com/en/contact/
http://www.example.com/en/contact/m
http://www.example.com/en/contact/m/
http://www.example.com/en/prices
http://www.example.com/en/prices/
http://www.example.com/en/prices/m
http://www.example.com/en/prices/m/
Check these links yourself and also other links as well.