Translation:After dinner my brother will watch a German movie.
Probably not what was meant here, but it's my understanding that "German movie" is Russian slang for a porn movie. At least I heard this phrase several times to indicate that meaning.
Yes, sometimes this phrase means porn. It's a euphemism mostly used in jokes.
Note to the moderators: In English, "is going to watch" = "will watch." The two expressions are completely synonymous, so either should be accepted.
I think the actual Russian equivalent for "going to" is собирается. Although, "going to" sounds more natural in English than "will watch."
I translated as "...will be watching ..."; isn't that a slightly better translation of an imperfective verb? If so, it should be accepted.
See also the question in this skill "She will be writing a letter in the evening", for which the given translation is Она будет писать письмо вечером.
I wrote ( my brother will watch a German film after dinner) why was it marked wrong !?