SLASH "/" in front of relative urls in RewriteRule: Correct Syntax?

Given:

RewriteCond %{HTTP_HOST} ^website.com$  
RewriteRule ^$ /de/home [L]

Question: examine the slash in front of de:

/de/home

de/home

What is the difference here and which is more advisable to use?

There is probably no difference in this context, because you are likely to be in the root directory.

Were you not in the root directory, but in a sub-folder like, say,

example.com/beta

, de/home could point to

example.com/beta/de/home

The starting slash makes sure the URL starts at the root directory, regardless where you are.