The diary entries are vanishing during gameplay. I played in German mode. Since I rarely used them I only able to say that they're gone at about day 7 or so (sorry for being so vague).
Seems to be a bug and it starts with day 1! Try to change the language in the settings, return to the game, open your diary (the text should appear in the different language) and change the language back to german. Then the text will show up again..