.htaccess can't add directory slash

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

重写--- www.example.com/?lang=en =＆gt; www.example.com/en/

RewriteCond %{QUERY_STRING} (^|&)lang=en($|&)
RewriteRule ^$ /en/? [L,R]

Answer 2

更新了测试代码

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]

请参阅浏览器地址栏并检查重定向是否正确。如果错误提供源链接和重定向链接。如果没问题，请使用以下代码。

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]

我已尝试使用以下链接的代码，并且其工作正常。

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/

自己检查这些链接以及其他链接。