The answer to the riddle is "slade".
Here's the breakdown:
- Mondvullende familie van de krulsla (Dutch): This translates to "moon-filling family of the curly lettuce".
- Krulsla (Dutch): This refers to curly lettuce, also known as frisée lettuce.
- Slade (English): This is a type of lettuce that is similar to frisée, with curly green leaves.
So, "slade" is the five-letter word that fits the description of being a relative of curly lettuce and having a name that evokes fullness like the moon.