I am, quite unfortunately, not the confident at all when it comes to translating from EN->JP, as all of my learning has been JP->EN, most of which is more formal speech. If you want, you can change up the Japanese text files and send me them, and I'll make sure to update the mod with the proper Japanese lines.
As for modding, it might help to make a copy of the original JP game, as then all of the variables and extra text in the editor will be in JP for easier understanding if JP is your first language, as I manually translated stuff as I went to help sort stuff out at a glance. The 'number of days passed' is variable 160, conception is variable 104, and pregnancy is variable 105. Alternatively, you could make the images separately and use a baby-bump version of the imouto as a new imageset, if the high learning curve of the editor is too much. There is a wiki for the editor here:
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