What is the ruling on buying ready-made dishes and sweets in non-Islamic countries (Japan) during the month of Ramadan, given the doubt concerning the presence of haram ingredients such as animal gelatin therein?

The basic principle concerning the slaughtered animals from countries with a pagan majority is that they are not lawful (halal). Consequently, gelatin extracted from them is forbidden (haram) because it is considered carrion (maytah). Unless something unlawful or impure (najis) enters it, the basic principle for anything else is permissibility, unless a Muslim becomes certain of its unlawfulness, or it is highly probable to him based on strong indicators.