I currently study three languages: English, French and German. At first, of these three, I had by far the best command of English. It is like a king, permitting no intrusion whatsoever.
But as the time went by and I got more and more used to the other languages, the state of English has begun to crumble down. For example, nowadays some German words tend to occur to me sooner than the English counterparts. Indeed, I was writing "English, French and Deutsch" at the beginning of this entry. Another example is concerned with the pronunciation. As some of you might know, the pronunciation of the letter r in French and German is very much different from that in English. As a part-time English teacher, I encourage students to pronounce English words and sentences while studying. Therefore, in my class, before I have them read them aloud, I often take the lead in the pronunciation. That's sometimes when the French or German r comes into play. It will sometimes make the student wonder why their teacher is trying to clear his throat in pronunciation.
I think this "erosion" of English will be settled, resulting in an order in my mind. The three are clearly separated but have meaningful associations. Hopefully.