Rewrite URL if file exists

I need to redirect web requests of the form /{language}-{country}/{file} to:

1. /{language}-{country}/{file} if it exists, otherwise
2. /{language}/{file} if it exists, otherwise
3. /en-US/{file}

The existing .htaccess fulfils requirements 1 and 3. What changes do I need to fulfil requirement 2?

.htaccess:

Options +FollowSymLinks
RewriteEngine On
RewriteCond $0 !i18n/en-US [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(i18n)/([^/]+)/(.*)$ $1/en-US/$3 [NC,L]

Update:

My new .htaccess meets the functional requirement but seems over-complicated. How can I simplify it?

# If not exists {language}-{country}, try {language}
RewriteCond $0 i18n
RewriteCond $0 !en-us
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond $0 ^(.*)(i18n)/(\w+)(-\w+)*/(.*)$
RewriteCond E:/webserver/app/$2/$3/$5 -f
RewriteRule ^(.*)(i18n)/(\w+)(-\w+)*/(.*)$ /app/$2/$3/$5 [L]

# If not exists {language}-{country} or {language}, try /en-us/
RewriteCond $0 i18n
RewriteCond $0 !en-us
RewriteCond %{REQUEST_FILENAME} !-f
Rewr开发者_Go百科iteCond $0 ^(.*)(i18n)/(\w+)(-\w+)*/(.*)$
RewriteCond E:/webserver/app/$2/$3/$5 !-f
RewriteCond E:/webserver/app/$2/en-us/$5 -f
RewriteRule ^(.*)(i18n)/(\w+)(-\w+)*/(.*)$ /app/$2/en-us/$5 [L]

Try this:

RewriteCond $0 .+
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond i18n/$1/$3 .+
RewriteCond %{DOCUMENT_ROOT}/%0 -f [OR]
RewriteCond i18n/en-US/$3 .+
RewriteRule ^i18n/([a-zA-Z]{2})-([a-zA-Z]{2})/(.*) %0 [L]

But this rule does only work is used in the .htaccess file in the document root.