If I plan one or more conditional moves, is it possible for my opponent to see which moves I have planned? I hope not, because it might give my opponent a hint towards the best move in that position. I could not find the answer to this question in this forum, so excuse me if it is already mentionned somewhere.
No, the planned moves are only visible to yourself.
Just for the sake of complete clarity since you asked - though not directly related - notes written to Malkovich log can however be viewed by your opponent under some circumstances. Malkovich log does not serve as private notes.
Private Notes in 2019!!
It might be in the queue behind Rengo (runs for cover)