Questions & Answers about wǒ gēn péngyou tán tiānqì.
Here 跟 most naturally means with. It introduces the person you are talking with:
我 跟 朋友 谈 天气
= I talk about the weather with a friend / with friends
Chinese 跟 can also mean and, so the sentence is a little ambiguous by itself, but in this pattern it is very commonly understood as with.
Because 谈 can take the topic directly.
So Chinese can say:
- 谈天气 = talk about the weather
- literally, something like discuss weather
This is normal in Mandarin. You do not need a separate word matching English about here.