How to fix cracks in cellar room?
You are correct that a surface cover will not resolve the issue and it's likely to disrupt flooring if done that way.
There are several commercial products for concrete foundation repairs, they are typically epoxy or polyurethane &quot;injections&quot; (this term may help you find some in your area but I do not know the German translation for the phrase &quot;concrete injection&quot;). The visible surface of the crack is temporarily sealed and then the epoxy applied with pressure so it pushes deep into the crack to seal it from the inside (or other side, if possible). In my experience these typically don't require drilling through the concrete so you can apply to the earth side, but variations may exist.
Read and follow the directions of whatever product you choose. Cracks are more commonly addressed in foundation walls so make sure your product is also rated for application to floor cracks.