I have asked this question long time ego but didn't get any answer
I wasn't sure whether to correct her or not. I chose not to. I speak to her in two languages and she used to mix them a little mainly because she couldn't say the word in the other language. Our native words seem to be more difficult and long so I was very happy she could express herself using words she knew. Now when she can pronounce almost everything she mixes less . Or she says it in both languages. I have noticed that some words she can translate which is great as I know she understands the content.
If there ever was a situation like you described: Do you want an apple? (in spanish),she answers: yes, apple (in dutch) ;I just replied yes it is an apple (in spanish).Sometimes she repeated after me in desired language straight away or she said the word later during play.I stopped focusing on that. I think that it takes a while to work out when to use which language.
This works for us very well