Translation:Kateřina and Matěj announced that they are getting married in the summer.
I'm confused: The point of this lesson appeared to be, if the principal clause has a "reporting" verb in the past form, then if the secondary clause is in the present form, the action is still past. But here, the action is present (or future).
I see the problem; maybe the solution is that the two announced something that (then) was in the future, and either still is or has happened in the meantime. In the first case i would use “are marrying,” in the second “were marrying.” But that does not really explain it.