What is the approved time for the 'Isha prayer in Germany, and is it permissible to pray the Tarawih with a mosque that has estimated the 'Isha time at 11 o'clock, and is it obligatory to repeat the 'Isha prayer if it was prayed at an early time?

One must wait for the disappearance of the red twilight to pray, as long as the timing is from a group of trustworthy people based on observation. It is not permissible to pray Isha before this time. Whoever prays it based on another calendar, their prayer is invalid and must be repeated. There is no blame on someone who finds it difficult to wait for Isha to combine the prayer with Maghrib, according to some scholars, due to need.