But an idiomatic translation of morbror is not 'maternal uncle' because we don't talk like that; whether an uncle is on your mum's or dad's side isn't important. However, if a person wanted to be specific, 'mum's/mother's brother' sounds far more like what a native speaker would say naturally than 'maternal uncle'.
"Maternal" = relating to mother, "paternal" = relating to father. This is why morbror is your mum's brother, farbror is your father's brother. Both are just "uncle" in English but if you're translating talking about a specific uncle of yours you need to know which side of the family they are on!
I am a native English speaker and I had never heard of a "paternal/maternal" uncle before this course. I don't doubt that it is correct but I think that among the majority of native speakers, this term would have to be explained when used. In general, I just say "uncle" but to distinguish which side of the family he is from, I have always said "my mother's brother" and this may seem awkward but it does sound very natural in the context of clarifying. Alternatively, some people might say "my uncle on my mother's side" but that is quite wordy. I have grown up in Australia but I don't recall ever noticing "paternal/maternal" in movies/TV/books.
For example, in telling a simple story, I would just refer to "my uncle" because that's primarily what he is but if the story had specific relevance to my mother or her family, I may refer to him initially as "my mother's brother."
Hope this helps. :)