Translation:Kateřina and Matěj announced that they are getting married in the summer.
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.
O.K. But who can tell me in every single sentence what the moderators are really meant? Because from where I see, in Czech sentence, there is nothing to point at it. P.S: Yes, it's better that way (I mean "in the summer"), but it's unnecessary. If you don't believe me, please, check "Grammarly" or different helpful your grammar checking program.
It is absolutely clear itis this summer, there is no doubt about it. If it were "že se budou brát v létě." then there would be a small posibility it just means it will be some summer and not winter. But here it is absolutely clear.
V pátek jedu domů. On Friday I go home. No doubt about that being the next Friday unless specified otherwise.