"Mangoes are fruits."
Translation:Maembe ni matunda.
direct translation doesn't always work out how you think. "-ko" are used for either locations or states of being, among other uses.
- Niko nyumbani - I am at home
- Uko wapi? - Where are you?
Also remember that just as the verb "to be" has many contexts, it translates to many forms in Swahili. "Is/Am/Are" in present tense is always "ni" (when used to describe what/who something/someone is)