How to rewrite from /page to /xx/page, using htaccess & apache only?

Question

Visualise a three step jump sporting event, with the first two steps perfect in place and the third step launching into air, never coming back and eventually a crash! That is what this question boils down to :)

STEP 1:WORKS serve /somepage?ln=xx when user navigates to /xx/somepage.

# if user inputs nice urls /xx/somepage  to serve page /someapge?ln=xx
RewriteRule ^([a-z][a-z])/(.*) /$2?ln=$1 [L]

STEP 2:WORKS go to the homepage when only language is given /xx/

# if language xx given but no page given? then redirect to its home /xx/home
RewriteRule ^([a-z][a-z])/?$ /$1/home [R=301,L]

STEP 3:CRASH

# if user inputs /somepage then redirect to default english: /en/somepage
# if user inputs /somepage?ln=xx redirect to nice url /xx/somepage
RewriteCond %{REQUEST_URI} !^/../ [NC]
RewriteRule ^(.*)$ /en/$1 [R=301,L]

The third rule is where we are stuck... PS: The redirection should work both for extensionless files somepage?ln=xx as well as files with extension somepage.abc?ln=xx where the extension can be any 2 or 3 char-word, or if easier, checked manually with the following extensions i use [.php .uu .u3c .vls]

Thanks a Thousand!

Answer 1

This does work (tested):

RewriteCond %{REQUEST_URI} !^/../ [NC]
RewriteRule ^(.*)$ /en$1 [R=301,L]

Apache Mod_Rewrite documentation, Rewrite Guide, and Advanced Rewrite Guide.

Edit:
Tested the above (it's the same as it was before); works on my Apache 2.2 server.

Edit 2:
Should be no problem, just need something like this:

# If just the language is specified (ie example.com/en)
RewriteCond %{REQUEST_URI} ^/..$ [NC]
RewriteRule ^(.*)$ $1/

# If no language subfolder, default to 'en'
RewriteCond %{REQUEST_URI} !^/../ [NC]
RewriteRule ^(.*)$ /en$1 [R=301]

# If no page specified, default to home.
RewriteCond %{REQUEST_URI} !^/../.+ [NC]
RewriteRule ^/(..) /$1/home.html [R=301]

# If no ln query, tack it on
RewriteCond %{QUERY_STRING} !ln= [NC]
RewriteRule ^/(..)/(.*) /$1/$2?ln=$1 [R=301]

Note: Be careful your 404 page works correctly. If it doesn't invalid links will end up in an infinite loop (ie, the 404 page wont work).
Note 2: If the user types in example.com/en, they will be redirected to example.com/en/en?ln=en, so be sure the tailing slash is in the URL, or else.
Note 3: If you want, you can drop the [R=301] from the last rule, then users will see example.com/en/home.html the request to the page will actually be example.com/en/home.html?ln=en. This will not work if your site uses GET requests though.

Edit 3:
Added another cond/rule pair to catch if someone types in just the language, without a trailing slash.