Lighttpd

This is a not a bug. The '.*' is a greedy match and will match the final '/' instead of '/?' matching the final '/'

Effectively, you have written:

url.rewrite-once=(
"^/files/(.*)$" => "/files.php?filename=$1",
"^/en/files/(.*)$" => "/files.php?filename=$1&lang=en" 
)

If your specific 'something' does not contain any slashes (and must contain at least one char), then you can use '([^/]+)' instead of (.*)

url.rewrite-once=(
"^/files/([^/]+)/?$" => "/files.php?filename=$1",
"^/en/files/([^/]+)/?$" => "/files.php?filename=$1&lang=en" 
)

My 'something' (slug) doesn't contain any forward slashes or backslashes, so I tried the rules you wrote. Unfortunately they don't work.

Since 'something' does not contain '/', you can try another alternative:

url.rewrite-once=(
"^/files/(.+?)/?$" => "/files.php?filename=$1",
"^/en/files/(.+?)/?$" => "/files.php?filename=$1&lang=en" 
)

I have interpreted the above as that you want to remove the trailing '/' if present. Please be much more descriptive of what "they don't work" actually means if that is not the case.

Another alternative is to simply remove the trailing '/' inside files.php.

The link //example.com/files/something/ isn't matched.
//example.com//files/something is matched
//example.com/en/files/some-thing/ isnt matched.
//example.com/en/files/some-thing is matched.

files.php creates a canonical tag without the trailing slash, however users are linking to the files with the trailing slash which gets redirected to the main page as its not matched within these rules.

UPDATE:
The last rules you suggested worked. It seems my last test was cached. Thank you!