In high school, about 15 years ago, I studied German for three years (my mother tongue is Dutch). The last eight year years I have studied and practiced Spanish and Catalan. And every time I try to talk German most words first come to me in Spanish or Catalan, and to remember the German words I really have to concentrate hard. This in fact already started the first year I studied Spanish. I heard that this happened to more people. My question are: Why does this happen? And: How can I fix this?
Note: I found these two related questions which focus on another aspect of learning another language: