.htaccess language based RewriteRule like example.com/en/ - apache

Question

I'm trying to write redirect directives in the .htaccess to forward internally all user requests like this:
Every request in a language folder should redirect to the requested file with the language query string:
example.com/en/contact.php -> example.com/contact.php?lang=en
Redirect any request without language path to a default language folder like this:
example.com -> example.com/en
Remove trailing slash if the address is entered with it:
example.com/en/ to example.com/en
For the folder projects, every request should lead to the view-project.php file with the respective query strings:
example.com/en/projects/test -> example.com/view-project.php?lang=en&path=test
Here is my attempt, but it's not working without trailing slash on a request like: http://www.example.com/de and is not redirecting http://www.example.com to a default language folder.
RewriteEngine On
RewriteRule ^(en|de)/(.*)$ $2?lang=$1 [L,QSA,NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^projects/([^/\.]+)/?$ view-project.php?path=$1 [QSA,L]
How can I achieve this?
This is possible a duplicate and I apologize for that. I searched everywhere and read about 100 posts, but I did't found what I'm looking for.

Answer 1

after struggling a while and with the help of someone else, here is the .htaccess file that works for me:
RewriteBase /example.com/
DirectorySlash Off
RewriteEngine On
RewriteOptions AllowNoSlash
RewriteRule ^$ de [R=301,L]
RewriteRule ^(de|en)/$ $1 [R=301,L]
RewriteRule ^(de|en)$ index.php?lang=$1 [L]
RewriteRule ^(de|en)/projects/(.+) view-project.php?lang=$1&path=$2 [L,QSA]
RewriteRule ^(de|en)/(.+) $2?lang=$1 [L,QSA]

Answer 2

Try:
RewriteEngine On
RewriteBase /mysite.com/
RewriteCond %{HTTP:Accept-Language} (en|de) [NC]
RewriteRule ^ /%1/index.php [L]
RewriteCond %{HTTP:Accept-Language} (en|de)
RewriteCond %{DOCUMENT_ROOT}%1%{REQUEST_URI} !-f
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteRule ^(en|de)/(.+)$ $2&lang=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^projects/([^/\.]+)/?$ view-project.php?path=$1 [QSA,L]
Edit:
The above was before the detailed explanation: After the detailed explanation, I've come up with this solution which is very similar to the solution the author has come up with while I was not aware about the edit explaining the situation:
DirectorySlash Off # disables mod_dir slash redirect
RewriteEngine On
RewriteBase /mysite.com/ # rewrites inside /mysite.com/
RewriteOptions AllowNoSlash # stops ignoring the directory redirects that redirect directories without slash, so by default: example.com/dir1 -> example.com/dir2 would be ignored. This is used because DirectorySlash is off
RewriteRule ^(en|de)\/projects\/(.+)$ view-project.php?lang=$1&path=$2 [QSA,L] # rule 4
RewriteRule ^(en|de)\/(.*)$ $2?lang=$1 [QSA,L] # rule 1
RewriteRule ^$ en [R=302,L] # rule 2
RewriteRule ^(en|de)\/$ $1 [R=302,L] # rule 3