There is only one expression that can be used as a filler in French and which phonetics is close enough to the German Lecker.

It is the expression d’accord, suggested by Teleporting Goat in his comment. D’accord meaning is close to Okay. It is commonly used either to tell you understand what was just said, or to ask if what you said is understood.

Here are some examples:

Emmanuel Macron: https://www.youtube.com/watch?v=OB4gkK4H3yE

Danyel Gérard: https://www.youtube.com/watch?v=2EF6huS24u8

Omar Sy: https://www.youtube.com/watch?v=BTWcQbUAZDQ

The other suggestion l’équerre is phonetically closer, but cannot be used as a filler where it would make absolutely no sense.