What is the advice for a Muslim residing in Germany regarding the difference in Fajr prayer timings (about half an hour) between two Sunni mosques in his city, one of which follows the Turkish calendar? And is he required to make up the days he fasted if he ate Suhoor after the first Fajr Adhan and before the later Fajr Adhan?

The fundamental principle for determining the times of prayer and fasting is knowing the Shariah-compliant signs, such as the appearance of the true dawn. If that is not possible, then there is no harm in following accredited calendars, provided they are specific to the country and their compilers are people of knowledge and trustworthiness. Whoever acts according to a calendar whose correctness he is confident about, due to its compilers being people of knowledge and religious piety, then his prayers and fasting are valid. The consideration for fasting is the entry of the time when the Adhan (definition) (call to prayer) is usually raised according to the calendar, not based on the prayer of the mosque's congregants. So if one abstains (from eating and drinking) relying on a calendar where the Fajr Adhan is later, believing it to be correct, then the fasting is valid and no make-up (qada') is required. It is recommended to investigate and seek out the calendar that is most congruent with correctness, and it is preferable for it to be in agreement with the Umm al-Qura calendar.