Is it permissible to file a lawsuit against the company to demand compensation for low wages, knowing that the wages violate German law and that I reluctantly agreed to the terms of the contract?

If an employment contract is concluded by mutual consent between the two parties, and it does not contain conditions that violate Allah's ruling, then it is not permissible to annul it, violate it, or invalidate it before its term expires, due to Allah's saying: (O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS), and the Prophet's (peace be upon him) saying: (Muslims are bound by their conditions). The basic principle is that conditions are binding on both parties, and violating them or attempting to violate them without a valid reason is not permissible. If a known wage has been agreed upon, it is not permissible to breach it or attempt to do so unless the condition itself is invalid in Islamic law. The one who is harmed by the work must abide by the agreed contract duration, and then has the option to continue or leave the job. Accordingly, as long as you have agreed to the contract terms, and there is nothing in them that violates Sharia, it is not permissible for you to demand compensation or an increase in wages, but you can leave the job after the contract term ends.