Is it permissible to perform the 'Isha and Fajr prayers with only a 30-minute interval between them in Belgium during the summer, given that the twilight and the star do not disappear between the two 'Isha prayers?

The time for the Isha prayer extends from the disappearance of the twilight (shafaq) until one third of the night. The length or shortness of the time does not affect it as long as the legal signs are present. The people in countries where the twilight does not disappear until late in the night must not pray Isha except after the twilight has vanished, because its disappearance marks the beginning of the Isha prayer time.