You’ll still mix languages, it’s normal. The brain is a wonderful organ, and will help you express yourself in the most efficient way possible. This means, you know what you want to say, you’ll pose your mouth to say it in the target language, but you’ve forgotten a word in the target so your brain will give you the word in the other language. Because it’s faster and more efficient than battling through trying to remember one word, lol!
Not what we want, but it is what it is.
English is my first language, I’ve been speaking Spanish for 20 years (less than half my life), learned a little German as a teen (relearning now), a little Japanese as a young adult, and actively learning Mandarin. I remember I was taking a Chinese class with a tutor and one other bilingual English/Spanish student. I was trying to put together a sentence talking about where my parents live. It started in Mandarin, the middle was English, and it ended in Spanish. Crazy funny!
Every time I want to practice a sentence in German talking about myself, I still want to say “wo.” The thing is, in German “wo” is pronounced “vo” and means “where.” I know “I” is “ich,” but yet my brain reverts.
It’ll happen. Just be aware of it, embrace it, have fun with it, and then refocus. Getting to a certain level in one language before starting another is no guarantee it won’t happen. If anything, the first target will be so engrained, it will pop up more when learning the second target.
For families that do more than one language, sometimes it is done situationally. For example, we have Mandarin mornings, but school is English, and afternoons and evenings with dad are in Spanish. Splitting the languages into situations or settings may be helpful if learning more than one language at a time.