To add some further context, all the Touhou games have been playable by Westerners for years due to fan translation patches. Even for the PC-98 games, the community has made preconfigured emulators packaged with all the games pre-patched to English so it's easy to play them without any technical knowledge. The reverse engineering project is orthogonal to this and is mainly intended to document the games' behavior as well as make them easier to mod. Having the source code available would make it possible to port the games to modern platforms, but they are pixel art 2D shooters so from a gameplay perspective there's not much of a benefit over playing them in an emulator.