Language areas of the brain are amazing. When I speak French, I occasionally, unintentionally, slip into Spanish ,as of the two, that is by far and away the better. I never slip into English from French nor Spanish. I, as almost all will, always do math in my main language, English. When I try to do math in Spanish, I need to actively translate in my head. When I am stuck for a word in Spanish, I mentally will scan English synonyms and sometimes come up with the Spanish word.
Idioms, as frequently they make little logical sense, need to be used to be retained.