You "speak [a language]" or you can "talk in [a language]". However, the first one will mean you can speak it, as in you are able to converse in it or use it, and the second implies you are having a conversation with someone in it. So you can say, "they are speaking Finnish" (they are speaking Finnish to eachother, to a group, or reading something out in it), "they speak Finnish" (they are able to speak Finnish), "they talk in Finnish" (i.e. [when doing one thing] they use Finnish), or "they are talking in Finnish" (they are talking to eachother using Finnish).
"They are speaking [a language other than English]" can also imply it is hard to understand them or they are talking nonsense.
I am inclined to agree with devalenteriel. In training for teaching English as a second language, we were told to discourage students from using constructions like "talk English" as they sound unnatural to so many native speakers, and many textbooks emphasise that it is a mistake. http://dictionary.cambridge.org/us/grammar/british-grammar/speak-or-talk http://www.englishgrammar.org/speak-talk-difference/
However, because the aim here is to learn Swedish rather than English and because some native speakers seem to be ok with using "talk" in this way (although it is much more common to use "speak"), we are accepting it as a translation.
Family relations are such that there are just so many synonyms, and each accepted solution has to be entered manually. So if you decide to add one synonym, you have to go through all sentences that have that word, and then add another accepted translation for every permutation of possible solutions to each of those sentences.
If Duolingo's system worked differently, I'm sure some words - like "folks" for "parents" - would have been at least brought up for consideration. But for now, there are practical reasons for sticking to the "main" translations, if you will.
We have a saying for this in Sweden, by the way. :) Kärt barn har många namn. - "A treasured child goes by many names."