"Valencia is a dream".
Valencia is a dream, there is absolutely nothing but positive things to say about the city, people, culture, etc. Particularly worth mentioning are the free public transport and free entry to almost all museums and cultural institutions for young people. The locals are very open and friendly if you make an effort to communicate in Spanish. It should be possible to study here without prior knowledge of the language, but it is definitely not recommended, as you will quickly get stuck in a German bubble and learn nothing about the local culture and people.
The Universitat Politècnica de València offers an extensive, free sports programme (very impressive and highly recommended, especially to get in contact with Spanish students). The courses in Spanish took some getting used to in the first few days, but you soon find your feet. Overall, compared to the University of Stuttgart, more is required during the semester (weekly submissions, projects, lab reports) and the engineering degree programme is generally much more practice-oriented. The exams are divided into two parts ("midterms" and "finals"), with the option of retaking both or just one (at the beginning of the next semester). This means that the effort required to prepare for the exam itself is comparatively less or equalised. In the exam itself, the tasks in Spanish must first be understood, but working on tasks in Spanish is easily feasible, as the language of engineers is the internationally valid maths anyway. However, the weekly submissions and projects are very time-consuming (formulating in Spanish and layout in scientific paper format).
It is advisable to take advantage of the Spanish language course at the UPV (free of charge, high teaching level, great opportunity to get to know other Erasmus students). It is also 4.5 ECTS, which is comparatively easy to achieve. It is definitely advisable to invest in learning the language, it literally opens the door to socialising with the locals.