Smell Problem in the Apartment

Not likely a good legal solution. No law specifically protects you from this and a civil suit puts the burden of proof on you to show that it makes life unbearable. It kind of hinges on rather vague, subjective factors. Bad cooking smells is not as clear cut  as say one using toxic chemicals.

It would likely cost you thousands in court cost, and/or lawyers and drag on for years and pit you against the neighbors. There is no such thing as small claims court in Germany. And no matter how clear you think the situation is, you very well might still lose. The judicial system in Germany for such things is very slow, expensive and often unpredictable especially arguing something is simply a nuisance. And if the neighbors lose they can appeal and it goes on a couple of more years in the next court.

One should really step up their efforts to make the neighbors realize how annoying it is for you without having to resort to legal action without a clear violation of law.

Being a landlord myself, I agree with Tom that legal action because of this is pretty hopeless - if you really can't tolerate the neighbour's behviour, better move elsewhere!

But before you do that, here is another option that Tom did not mention above:

The landlord is wrong in saying he cannot do anything. In fact, if the situation is becoming unbearable, he HAS TO do something - because you pay for having a reasonable stay there and he owes you this!

To force him to do something, you can withhold a portion of the rent (after a written announcement that you would do so). The amount withheld must be proportionate to the size of the problem - this is a difficult judgement (you withhold too little and the landlord just laughs it off, you withhold too much and he can terminate your rental contract!), so you should consult a lawyer or tenants' association before making a decision! Also, you should make a detailed diary of the issues (with witnesses if possible). If the other tenants do the same, together with you, your chances of success multiply!