Same for me.
My mother tongue is French. When I need to translate English to French, I have to really think hard about it, because when you read or listen, you get the context of what is said, the whole meaning, but you do not translate in your mind word word by word what you've just been told/read. You just don't think twice .
When I speak English I do not think in French first, it just comes (within my range of vocabulary, that is. I use English every single day at work as my bosses do not speak French). But some days when I'm tired, or if I have not used a word for a long time, I will just forget it (which is why it's important to maintain a language learned by reading, speaking etc).
Mika has a fluent level of French, his accent is very light (actually to me he's talking like he's nearly always lived in France), he speaks fast, you can see he's at ease with the language. Just he must have not learned/forgotten some words as he does not talk it 24/7... Add to it if he's tired, the words won't come back as easily as usual...
And regarding the mix-up of two languages, I'm the QUEEN! People can get bothered at me because I will mix so much English/French, but I can't help it, some words in French just don't come back to me, English has replaced them! Sometimes I wonder: what would I do at a job interview, that would not give a good impression.... .