HeebieJeebies you are absolutely wrong. Our family is about as bilingual practising as it is humanly possible to be. The children have been fully, deliberately and actively immersed in both languages from birth. Ironically enough I have to use DH's language exclusively at work, and he has to use mine.
We speak the minority language (English) almost exclusively at home (unless the house is full of the kids friends), it is our family language and the kids also read and write it, and I read to them every night. The rest of the children's lives (all of their friends, local school and before that local village Kindergarten, hobbies, socialising etc) is in the community language (German).
English people think my kids are monolingual English and Germans think my kids are monolingual Germans. They reply in the language they are addressed in (unless the person is speaking it very badly - speak really terrible German or English to them and they won't recognise it and will guess from context which language to speak).
They speak both languages as natives, that is utterly separate to being able to simultaneously translate. They've grown up with both languages from the day they were born, and when very small couldn't even tell you which language was which, they just understand and use the language automatically.
Very few people can truly simultaneously translate, even those who have grown up with two languages. It requires a certain type of memory, and exactly the same vocabulary in both languages (i.e. not an association of one language with one topic, which happens if you go to a monolingual school and your exposure to your other language/ s is domestic or social).
Many people who have grown up speaking multiple languages mix them in conversation with their family and anyone else who has also grown up with the same language mix, because one language will lend itself better to one topic than another. If you've only learnt maths in Polish it will be hard to explain it in English, no matter how well you can discuss literature and music, say, in English.
A lot of people who have grown up genuinely bilingual don't translate. That is a key - if both languages are native to you, you never naturally translate between them. Only learners of a language do that generally.