"Mount Kilimanjaro is visible"
Translation:Mlima Kilimanjaro unaonekana
kuonekana is "to be visible"
There's a small group of verbs that get -ekana/ikana instead of just -eka/ika. It kind of looks to me that there's a bit of a link to the concept of "widely" or "by people at large", but it's best to simply remember these verbs as you come across them
kujulikana = to be well-known
kupatikana = to be available
kutakikana = to be needed/wanted
kushindikana = to be unsuccessful
kuwezekana = to be possible
kuonekana = to be visible
kukosekana = to be missing/lacking
kusemekana = to be purported/claimed
cf. kusemeka = to be utterable/speakable/uttered/spoken
If I understand correctly, the -me- tense is used in Swahili (apart from the past perfect) to describe 'something/someone' as the result of an action, and this 'something/someone' is still in a certain state because of this action:
Nimelewa, because I've been drinking
Umechoka, because you've been running
The visibility of Mt. Kilimanjaro is obviously not the result of an action. Therefore umeonekana should simply mean 'has been visible', i.e. just the past perfect.