1
She can’t talk in English.
She can’t speak English.
Use speak + language when talking about someone’s ability to use a language: ‘She speaks English very well.’ ‘I can’t speak German.’
The purple sentence is possible if it means that she cannot have a conversation in English. However, when referring to general language ability, speak is the normal choice.
2
I’d like to talk you about something.
I’d like to talk to you about something.
Use talk to/with someone about something: ‘I need to talk to you about the problem.’ ‘She was talking with a friend about her new job.’
3
We only talked of the weather.
We only talked about the weather.
Use talk about something to introduce the subject of a conversation: ‘We talked about the weather.’
When mentioning a person, use talk to/with someone about something: ‘I talked to my neighbour about the weather.’